Exhaust Gas Purifying Device for Internal Combustion Engine

ABSTRACT

An NO X  occluding and reducing catalyst  7  is arranged in an exhaust gas passage  2  of an engine  1  so as to occlude, reduce and purify NO X  contained in an exhaust gas. An H2 sensor  33  is arranged in the exhaust gas passage on a downstream side of the NO X  occluding and reducing catalyst  7  and a hydrogen component concentration of the exhaust gas is detected. When an amount of NO X  occluded in the NO X  occluding and reducing catalyst is increased to a predetermined value, an electronic control unit (ECU)  30  of the engine operates the engine at a rich air-fuel ratio, and a regenerating operation, by which a rich air-fuel ratio exhaust gas is supplied to the NO X  occluding and reducing catalyst, is executed so as to reduce and purify NO X  which is occluded in the NO X  occluding and reducing catalyst. At the time of executing the regenerating operation, when the H 2  sensor  33  detects hydrogen components in the exhaust gas, ECU  30  finishes the regenerating operation. Due to the foregoing, it is possible to accurately judge the time for terminating the regenerating operation of the NO X  occluding and reducing catalyst.

TECHNICAL FIELD

The present invention relates to an exhaust gas purifying device for aninternal combustion engine. More particularly, the present inventionrelates to an exhaust gas purifying device for an internal combustionengine in which an NO_(X) occluding and reducing catalyst is used.

BACKGROUND ART

An exhaust gas purifying device for an internal combustion engine whichincludes an NO_(X) occluding and reducing catalyst is well known in theart. An NO_(X) occluding and reducing catalyst occludes NO_(X)components in the exhaust gas when the air-fuel ratio of exhaust gasflowing into the catalyst is lean and reduces NO_(X) occluded in thecatalyst by reduction using the reducing component in the exhaust gaswhen the air-fuel ratio of the exhaust gas flowing into the catalyst isrich or stoichiometric air-fuel ratio. In this connection, the term“occlusion” used in this specification includes both the concept ofadsorption and absorption.

When the air-fuel ratio is lean, an NO_(X) occluding and reducingcatalyst occludes NO_(X) components, which are contained in the exhaustgas, in the occlusion material such as BaO, in the form of nitric acidions. Therefore, when an amount of NO_(X), which has been occluded in anNO_(X) occluding and reducing catalyst, is increased, the occlusionmaterial is saturated with NO_(X), and it becomes difficult for catalystto occlude NO_(X) contained in the exhaust gas.

Therefore, in the exhaust gas purifying device in which an NO_(X)occluding and reducing catalyst is used, every time an amount of NO_(X)occluded in the NO_(X) occluding and reducing catalyst is increased, arich spike operation is conducted in which exhaust gas of a richair-fuel ratio is supplied to the NO_(X) occluding and reducing catalystfor a short period of time, so that NO_(X) occluded by the catalyst canbe reduced and purified. This rich-spike technique is described in, forexample, Japanese Patent Publication No. 2600492.

When the air-fuel ratio of the exhaust gas becomes a stoichiometricair-fuel ratio or a rich air-fuel ratio, an amount of reducingcomponents such as CO and an amount of HC components contained in theexhaust gas are sharply increased. NO_(X) desorbed from the occlusionmaterial of the NO_(X) occluding and reducing catalyst reacts with CO,HC and so forth and is reduced to _(N2) and, therefore, the amount ofNO_(X) occluded in the NO_(X) occluding and reducing catalyst decreases.Thus, it becomes possible for the NO_(X) occluding and reducing catalystto occlude NO_(X) again under the condition of a lean air-fuel ratio.

As described above, the rich spike operation, which is conducted for thereduction and purification of NO_(X) occluded in the NO_(X) occludingand reducing catalyst (this reduction and purification of NO_(X)occluded in the NO_(X) occluding and reducing catalyst is hereinafterreferred to as “regeneration of the NO_(X) occluding and reducingcatalyst”) is accompanied by the rich air-fuel ratio operation of anengine and also accompanied by the addition of fuel or a reducing agentinto the exhaust gas. Accordingly, if the regenerating operation iscontinued even after the regenerating operation of the NO_(X) occludingand reducing catalyst has been completed, the fuel consumption of theengine is increased, or the emission is deteriorated by the discharge ofthe reducing agent.

Therefore, it is necessary to judge that the regeneration of the NO_(X)occluding and reducing catalyst has been completed, that is, it isnecessary to judge that all of the occluded NO_(X) has been reduced andpurified, and it is also necessary to complete the regeneratingoperation when the regeneration has been completed.

For this purpose, Japanese Patent Publication No. 2692380 discloses anexhaust gas purifying device in which an O₂ sensor is disposed on thedownstream side of the exhaust gas passage of the NO_(X) occluding andreducing catalyst. The output of the O₂ sensor changes according towhether the air-fuel ratio of exhaust gas is rich or lean. According tothe output of this O₂ sensor, the completion of the regeneration of theNO_(X) occluding and reducing catalyst is judged.

As described above, when exhaust gas of a rich air-fuel ratio flows intothe NO_(X) occluding and reducing catalyst at the time of a rich spikeoperation, components such as HC, CO and so forth contained in theexhaust gas are consumed in order to reduce NO_(X) occluded in theNO_(X) occluding and reducing catalyst. In the other words, HC, CO andso forth are oxidized by oxygen contained in NO_(X). Therefore, evenwhen an exhaust gas of a rich air-fuel ratio flows into the NO_(X)occluding and reducing catalyst while NO_(X) is being reduced by theNO_(X) occluding and reducing catalyst, HC, CO and so forth are oxidizedby oxygen, which has been desorbed from the NO_(X) occluding andreducing catalyst. Therefore, the air-fuel ratio of exhaust gas on thedownstream side of the NO_(X) occluding and reducing catalyst ismaintained at the stoichiometric air-fuel ratio. Then, when theregeneration of the NO_(X) occluding and reducing catalyst is completedand all NO_(X) is reduced, the components of HC, CO and so forthcontained in the exhaust gas are not oxidized by the NO_(X) occludingand reducing catalyst. This causes the air-fuel ratio of exhaust gas onthe downstream side of the NO_(X) occluding and reducing catalyst tobecome a rich air-fuel ratio which is the same as that on the upstreamside.

That is, the air-fuel ratio of exhaust gas on the downstream side of theNO_(X) occluding and reducing catalyst does not become a rich air-fuelratio right after the start of the rich spike operation but it ismaintained at a value close to the stoichiometric air-fuel ratio, andwhen all of NO_(X) occluded in the NO_(X) occluding and reducingcatalyst has been reduced, that is, only when the regeneration of theNO_(X) occluding and reducing catalyst has been completed, the air-fuelratio of exhaust gas on the downstream side of the NO_(X) occluding andreducing catalyst is changed to a rich air-fuel.

According to the apparatus disclosed in Japanese Patent Publication No.2692380, the completion of the regeneration of the NO_(X) occluding andreducing catalyst is judged as follows. An output of an O₂ sensor, whichis disposed on the downstream side of the NO_(X) occluding and reducingcatalyst, is monitored. When it is detected that an output of an O₂sensor is changed from a stoichiometric air-fuel ratio to a richair-fuel ratio at the time of the rich spike operation, it is judgedthat regeneration of the NO_(X) occluding and reducing catalyst iscompleted.

In this connection, hydrogen has a high reducing capacity compared withCO. Therefore, when an appropriate amount of hydrogen is supplied to theNO_(X) occluding and reducing catalyst at the time of reducing andpurifying the NO_(X) occluded in the NO_(X) occluding and reducingcatalyst, a reducing rate of NO_(X) occluded in the NO_(X) occluding andreducing catalyst is increased, and NO_(X) can be effectively reducedand purified in a short period of time.

It is known that hydrogen is generated by the combustion of fuel in anengine when the air-fuel ratio is rich. Further, a method is known inwhich hydrogen from another source is added to exhaust gas in additionto hydrogen generated in the process of a rich air-fuel ratio operationof a usual engine.

For example, Japanese Unexamined Utility Model Publication (Kokai) No.2002-47919 discloses a method in which the NO_(X) occlusion capacity ofthe NO_(X) occluding and reducing catalyst is judged by utilizing areducing capacity of hydrogen which is a strong reducing agent.

As hydrogen has a high reducing capacity, if NO_(X) is occluded in thecatalyst, hydrogen components in the exhaust gas flowing into the NO_(X)occluding and reducing catalyst are consumed by reacting with NO_(X)occluded in the catalyst and hydrogen components do not flow out ontothe downstream side of the NO_(X) occluding and reducing catalyst aslong as NO_(X), occluded in the NO_(X) occluding and reducing catalyst,exists.

Accordingly, in the case where the exhaust gas of a rich air-fuel ratiocontaining hydrogen is supplied to the NO_(X) occluding and reducingcatalyst, a point of time at which hydrogen starts flowing out into theexhaust gas on the downstream side of the NO_(X) occluding and reducingcatalyst can be thought to be a point of time at which all of the NO_(X)occluded in the NO_(X) occluding and reducing catalyst has been reducedand purified. Accordingly, a period of time from the start of the richspike operation to the detection of hydrogen components on thedownstream side corresponds to the amount of NO_(X) occluded in theNO_(X) occluding and reducing catalyst. Accordingly, it can be judgedthat the longer this period of time, the larger the amount of NO_(X)occluded in the NO_(X) occluding and reducing catalyst.

The technique disclosed in Japanese Unexamined Patent Publication(Kokai) No. 2002-47919 utilizes the phenomenon explained above forjudging the completion of the regeneration of the NO_(X) occluding andreducing catalyst.

In Japanese Unexamined Patent Publication (Kokai) No. 2002-47919, H₂sensors for detecting hydrogen in the exhaust gas are disposed on boththe upstream side and the downstream side of the NO_(X) occluding andreducing catalyst. At the time of reducing and purifying the occludedNO_(X), according to a difference of time from the detection of hydrogenby the upstream H₂ sensor to the detection of hydrogen by downstream H₂sensor, it is judged whether or not an amount of NO_(X) occluded in theNO_(X) occluding and reducing catalyst is decreased, that is, it isjudged whether or not the NO_(X) occluding and reducing catalyst isdeteriorated.

Japanese Unexamined Patent Publication (Kokai) No. 2003-120383 disclosesa method for controlling the air-fuel ratio based on the concentrationof hydrogen component in the exhaust gas of the internal combustionengine. In this publication, in order to prevent the occurrence of acase in which an error is caused in an output of an oxygen sensordisposed on the downstream side of a three way catalyst by hydrogengenerated by the three way catalyst at the time of operation of a richair-fuel ratio of a stoichiometric air-fuel ratio and, in order toprevent the occurrence of a case in which air-fuel ratio control isconducted according to the output of the oxygen sensor is affected bythe error, a hydrogen sensor is disposed on the downstream side of thethree way catalyst and air-fuel ratio control is corrected based on theconcentration of hydrogen components in the exhaust gas measured by thehydrogen sensor.

As explained above, in the case where the NO_(X) occluding and reducingcatalyst is regenerated by the rich spike operation, in order to preventan increase in the fuel consumption of an engine, it is necessary toaccurately judge that the regeneration of the NO_(X) occluding andreducing catalyst has been completed.

However, as described in Japanese Patent Publication No. 2692380, whenthe time of the completion of the regeneration of the NO_(X) occludingand reducing catalyst is judged at the time of a rich spike operationbased on the output of the oxygen sensor disposed on the downstream sideof the NO_(X) occluding and reducing catalyst, it is difficult toaccurately judge the time of the completion of the regeneration in somecases.

As explained later, in order to reduce NO_(X) by HC, CO and so forth inthe exhaust gas at the time of a rich spike operation, it is necessaryfor the catalyst component such as platinum (Pt) contained in the NO_(X)occluding and reducing catalyst to function as a reducing catalyst.However, in the case where a large amount of HC components are containedin the exhaust gas, the HC components are adsorbed onto a surface of thecatalyst. Therefore, the surface of the catalyst is covered with the HCcomponents, and it becomes difficult for the catalyst components tofunction as a reducing catalyst, that is, a problem of covering iscaused on the catalyst components.

Therefore, in the case where a large amount of HC components arecontained in the rich air-fuel ratio exhaust gas supplied to the NO_(X)occluding and reducing catalyst at the time of rich spike operation, aportion of NO_(X) occluded in the NO_(X) occluding and reducing catalystand a portion of HC components contained in the exhaust gas, withoutreacting each other, flow out to the downstream side of the NO_(X)occluding and reducing catalyst.

When HC components contained in the rich air-fuel ratio exhaust gaspasses through the NO_(X) occluding and reducing catalyst withoutreacting with NO_(X) and flow out onto the downstream side of thecatalyst as described above, the oxygen sensor disposed on thedownstream side of the catalyst judges that the air-fuel ratio of theexhaust gas becomes rich. Therefore, when the completion of theregeneration of the NO_(X) occluding and reducing catalyst is judgedaccording to the output of the oxygen sensor disposed on the downstreamside of the catalyst as described in Japanese Patent Publication No.2692380, problems may occur. Namely, although the regeneration is notactually completed, the air-fuel ratio of the exhaust gas detected bythe oxygen sensor becomes a rich air-fuel ratio, and it is erroneouslyjudged that the regeneration is completed, and the rich spike operationis terminated. When the regenerating operation is terminated while theregeneration of the NO_(X) occluding and reducing catalyst has not beensufficiently carried out, the NO_(X) occluding and reducing catalyst hasto resume the occlusion of NO_(X) before the NO_(X) occlusion capacitythereof has not been sufficiently regenerated. This results ininsufficient exhaust gas purification and deterioration of emission.

As described above, supplying hydrogen components is effective for theregeneration of the NO_(X) occluding and reducing catalyst. However,when the NO_(X) occluding and reducing catalyst is regenerated bysupplying hydrogen components to the NO_(X) occluding and reducingcatalyst, unless an appropriate amount of hydrogen components aresupplied, it is difficult to sufficiently regenerate the NO_(X)occluding and reducing catalyst, and the exhaust gas can not besufficiently purified.

According to the techniques disclosed in Japanese Unexamined UtilityModel Publications (Kokai) No. 2002-47919 and 2003-120383, hydrogencomponents in the exhaust gas are detected by the H₂ sensors. However,no consideration is given to controlling an amount of H₂ componentscontained in the exhaust gas supplied to the catalyst at the time ofregenerating the NO_(X) occluding and reducing catalyst.

DISCLOSURE OF THE INVENTION

In view of the problems explained above, an object of the presentinvention is to provide an exhaust gas purifying device for an internalcombustion engine in which an NO_(X) occluding and reducing catalyst isappropriately regenerated even in the case where a relatively largeamount of HC is contained in exhaust gas so that the exhaust gas can besufficiently purified.

In order to achieve the above object, according to the inventiondescribed in claim 1, there is provided an exhaust gas purifying devicefor an internal combustion engine comprising: an NO_(X) occluding andreducing catalyst disposed in an exhaust passage of an internalcombustion engine, the NO_(X) occluding and reducing catalyst occludingNO_(X) contained in exhaust gas by one of the absorption and theadsorption or by both the absorption and the adsorption when an air-fuelratio of the exhaust gas flowing into the catalyst is lean, and reducingand purifying the occluded NO_(X) with a reducing component contained inthe exhaust gas when an air-fuel ratio of the exhaust gas is astoichiometric air-fuel ratio or a rich air-fuel ratio; and an H₂ sensordisposed in at least one of the exhaust gas passages on the inlet sideand the outlet side of the NO_(X) occluding and reducing catalyst fordetecting a hydrogen component concentration of the exhaust gas, whereinthe exhaust gas purifying device executes a regenerating operation inwhich the exhaust gas of a rich air-fuel ratio or a stoichiometricair-fuel ratio is supplied to the NO_(X) occluding and reducing catalystfor a predetermined period of time when the NO_(X) occluding andreducing catalyst is to reduce and purify the NO_(X) occluded in theNO_(X) occluding and reducing catalyst and, during the regeneratingoperation, the exhaust gas purifying device controls the air-fuel ratioof the exhaust gas flowing into the NO_(X) occluding and reducingcatalyst based on the hydrogen component concentration in the exhaustgas detected by the H₂ sensor.

Namely, according to the invention described in claim 1, an air-fuelratio of exhaust gas is controlled according to the hydrogen componentconcentration detected by the H₂ sensor disposed on at least one of theupstream side and the downstream side of the NO_(X) occluding andreducing catalyst.

For example, as described later, when the hydrogen componentconcentration detected by the H₂ sensor disposed on the downstream sideof the NO_(X) occluding and reducing catalyst is used, it is possible tojudge the completion time of the regenerating operation of the NO_(X)occluding and reducing catalyst. Therefore, accordingly, theregenerating operation can be terminated at a time exactly the same asthe completion of the regeneration of the NO_(X) occluding and reducingcatalyst and the air-fuel ratio of the exhaust gas can be returned to alean air-fuel ratio. Therefore, the NO_(X) occluding and reducingcatalyst can be appropriately regenerated.

The hydrogen component concentration in the exhaust gas changes inaccordance with the air-fuel ratio of the exhaust gas. Accordingly, whenthe air-fuel ratio of the exhaust gas is controlled based on thehydrogen component concentration in the exhaust gas detected by the H₂sensor disposed on the upstream side of the NO_(X) occluding andreducing catalyst, for example, the hydrogen component concentration inthe exhaust gas flowing into the NO_(X) occluding and reducing catalystcan be controlled to be an appropriate value and an appropriate amountof hydrogen components can be supplied to the NO_(X) occluding andreducing catalyst. Therefore, the NO_(X) occluding and reducing catalystcan be appropriately regenerated.

According to the invention described in claim 2, there is provided anexhaust gas purifying device for an internal combustion engine accordingto claim 1, wherein the H₂ sensor is arranged in an exhaust gas passageon an outlet side of the NO_(X) occluding and reducing catalyst and, atthe time of executing the regenerating operation, a time for terminatingthe regenerating operation is judged according to a hydrogen componentconcentration in the exhaust gas detected by the outlet side H₂ sensor.

According to the invention described in claim 2, the H₂ sensor isdisposed at least on the outlet side (the downstream side) of the NO_(X)occluding and reducing catalyst. When the rich spike operation forconducting the regeneration of the NO_(X) occluding and reducingcatalyst is started, as the air-fuel ratio of the exhaust gas flowinginto the NO_(X) occluding and reducing catalyst is made to be a richair-fuel ratio, the hydrogen component concentration in the exhaust gasis increased.

However, as the reducing capability of hydrogen is very strong, whenhydrogen flows into the NO_(X) occluding and reducing catalyst, it candirectly reduce NO_(X) without the assistance of the reducing catalyst.Therefore, even if a relatively large amount of HC components arecontained in the exhaust gas and the covering of the catalyst, in whicha catalyst surface is covered with HC, occurs and the function of thereducing catalyst is deteriorated, hydrogen contained in the exhaust gasis consumed by reacting with NO_(X).

Therefore, even in the case where the covering is caused on the catalystby HC components in the exhaust gas, as long as the regeneration of theNO_(X) occluding and reducing catalyst is not completed, hydrogen is notdetected by the H₂ sensor on the downstream side of the catalyst.

In the present invention, in the case where hydrogen components aredetected by the H₂ sensor disposed on the downstream side of the NO_(X)occluding and reducing catalyst in the process of the rich spikeoperation, it is judged that the regeneration of the NO_(X) occludingand reducing catalyst has been completed, and the regenerating operationis terminated.

Due to the foregoing, in the present invention, even in the case where arelatively large amount of HC components are contained in the exhaustgas, it is possible to accurately judge that the regeneration of theNO_(X) occluding and reducing catalyst has been completed, and theregenerating operation can be terminated. Therefore, it is possible toprevent the occurrence of a problem in which the exhaust gas purifyingcapacity is deteriorated when the regenerating operation is terminatedbefore the regeneration of the NO_(X) occluding and reducing catalystcompletes. Further, it is possible to prevent the occurrence of aproblem in which the regenerating operation is continued even after thecompletion of the regeneration and the fuel consumption is increased.Therefore, according to the present invention, the NO_(X) occluding andreducing catalyst can be appropriately regenerated.

According to the invention described in claim 3, there is provided anexhaust gas purifying device for an internal combustion engine accordingto claim 1, wherein the H₂ sensor is arranged in at least an exhaust gaspassage on the outlet side of the NO_(X) occluding and reducingcatalyst, the regenerating operation includes operations for firstsupplying the exhaust gas of a rich air-fuel ratio to the NO_(X)occluding and reducing catalyst and then supplying the exhaust gas of astoichiometric air-fuel ratio to the NO_(X) occluding and reducingcatalyst and, the time at which the exhaust gas air-fuel ratio isswitched from the rich air-fuel ratio to the stoichiometric air-fuelratio is determined in accordance with the hydrogen componentconcentration detected by the outlet side H₂ sensor.

That is, in the invention described in claim 3, the regeneratingoperation is conducted by switching the air-fuel ratio into two steps ofa rich air-fuel ratio and a stoichiometric air-fuel ratio. For example,in some cases, a high occlusion type NO_(X) occluding and reducingcatalyst in which the NO_(X) occlusion capacity thereof is enhanced isused. The high occlusion type NO_(X) occluding and reducing catalyst isan NO_(X) occluding and reducing catalyst in which an amount of NO_(X)occluded per unit volume is greatly increased by using an occlusionmaterial having a high affinity with NO_(X). In the high occlusion typeNO_(X) occluding and reducing catalyst, as the affinity of the occlusionmaterial with NO_(X) is high, after a relatively large amount of NO_(X)has been desorbed at the initial stage of regeneration, a rate ofdesorption of NO_(X) is decreased. Therefore, in order to completelyregenerate the NO_(X) occluding and reducing catalyst, it is necessaryto conduct the regenerating operation over a long period of time.

Therefore, in the regenerating operation of the high occlusion typeNO_(X) occluding and reducing catalyst, in order to suppress an increasein the fuel consumption, exhaust gas of a rich air-fuel ratio issupplied to the NO_(X) occluding and reducing catalyst at the initialstage of the regenerating operation so as to desorb a relatively largeamount of NO_(X) and the catalyst is reduced and purified. After that,the air-fuel ratio of the exhaust gas is switched over to astoichiometric air-fuel ratio and the NO_(X) occluding and reducingcatalyst is completely regenerated over a relatively long period oftime.

In this case, while a relatively large amount of NO_(X) is beingdesorbed from the NO_(X) occluding and reducing catalyst at the initialstage of the rich spike operation, all hydrogen components contained inthe rich air-fuel ratio exhaust gas are consumed by reacting withNO_(X), however, when the desorption of the initial stage is finishedand a rate of desorption of NO_(X) is decreased, a portion of thehydrogen components contained in the exhaust gas flow out onto thedownstream side of the NO_(X) occluding and reducing catalyst withoutreacting with NO_(X).

In the present invention, when the hydrogen components are detected bythe downstream side H₂ sensor, the air-fuel ratio of the exhaust gas ischanged over from the rich air-fuel ratio to the stoichiometric air-fuelratio. Therefore, even when the high occlusion type NO_(X) occluding andreducing catalyst is used, the NO_(X) occluding and reducing catalystcan be appropriately regenerated without continuing the rich air-fuelratio operation for an unnecessary long period of time at the time ofthe regenerating operation.

In this connection, when the air-fuel ratio of the exhaust gas ischanged to the stoichiometric air-fuel ratio, hydrogen is seldomgenerated. Therefore, hydrogen is not detected by the downstream side H₂sensor.

According to the invention described in claim 4, there is provided anexhaust gas purifying device for an internal combustion engine accordingto claim 1, wherein the H₂ sensor is arranged in an exhaust gas passageon the inlet side of the NO_(X) occluding and reducing catalyst, and atthe time of executing the regenerating operation, an air-fuel ratio ofthe exhaust gas flowing into the NO_(X) occluding and reducing catalystis controlled so that a hydrogen component concentration in the exhaustgas detected by the inlet side H₂ sensor can be a predetermined targetvalue.

That is, in the invention described in claim 4, the H2 sensor disposedon the inlet side (the upstream side) of the NO_(X) occluding andreducing catalyst detects a concentration of the hydrogen componentsflowing into the NO_(X) occluding and reducing catalyst, and theair-fuel ratio of the exhaust gas is controlled by feedback control insuch a manner that the detected concentration of the hydrogen componentsbecomes a target value.

Therefore, when the regenerating operation is conducted, an appropriateamount of hydrogen can be supplied to the NO_(X) occluding and reducingcatalyst at all times, and the NO_(X) occluding and reducing catalystcan be appropriately regenerated.

According to the invention described in claim 5, there is provided anexhaust gas purifying device for an internal combustion engine accordingto claim 4, wherein the target value of the hydrogen componentconcentration is high at the time of starting the regeneratingoperation, and then the target value of the hydrogen componentconcentration is gradually decreased with the lapse of time.

That is, in the invention described in claim 5, the concentration of thehydrogen components flowing into the NO_(X) occluding and reducingcatalyst is set according to the NO_(X) desorbing rate at which NO_(X)is desorbed from the NO_(X) occluding and reducing catalyst at the timeof the regenerating operation.

When the air-fuel ratio of the exhaust gas flowing into the NO_(X)occluding and reducing catalyst is changed from lean to rich at the timeof the regenerating operation, an amount of NO_(X), which is desorbedfrom the NO_(X) occluding and reducing catalyst, is large immediatelyafter the air-fuel ratio is changed over to a rich air-fuel ratio. Then,the amount of NO_(X) is decreased with the lapse of time.

Accordingly, the hydrogen component concentration in the exhaust gasflowing into the NO_(X) occluding and reducing catalyst at the time ofthe regenerating operation is set high at the time of starting theregenerating operation and then is gradually decreased. Due to theforegoing, the hydrogen components can be supplied according to anamount of NO_(X) desorbed from the NO_(X) occluding and reducingcatalyst, and the NO_(X) occluding and reducing catalyst can beappropriately regenerated.

According to the invention described in claim 6 there is provided anexhaust gas purifying device for an internal combustion engine accordingto claim 2, wherein according to the lapse of time from the start of theregenerating operation to the end of the regenerating operation which isjudged according to the hydrogen component concentration in the exhaustgas detected by the downstream side H₂ sensor, a degree of deteriorationof the NO_(X) occluding and reducing catalyst is judged.

That is, in the invention described in claim 6, a completion of theregeneration of the NO_(X) occluding and reducing catalyst is judgedbased on the hydrogen component concentration detected by the downstreamside H₂ sensor. At the same time, according to the lapse of time fromthe start of the regenerating operation to the completion of theregeneration of the NO_(X) occluding and reducing catalyst, a degree ofdeterioration of the NO_(X) occluding and reducing catalyst is judged.

The required time from the start to the completion of the regeneratingoperation corresponds to an amount of the occluded NO_(X) of the NO_(X)occluding and reducing catalyst. When the NO_(X) occluding and reducingcatalyst is deteriorated and an amount of NO_(X) capable of beingoccluded is decreased accordingly, the required time for the completionof regeneration is shortened.

Therefore, in the case where the required time is decreased to apredetermined judgment value, it is possible to judge that the NO_(X)occluding and reducing catalyst has been deteriorated.

Due to the foregoing, according to the present invention, while theNO_(X) occluding and reducing catalyst is being appropriatelyregenerated, a degree of deterioration of the NO_(X) occluding andreducing catalyst can be accurately judged.

According to the invention described in claim 7, there is provided anexhaust gas purifying device for an internal combustion enginecomprising an NO_(X) occluding and reducing catalyst disposed in anexhaust passage of an internal combustion engine, the NO_(X) occludingand reducing catalyst occluding NO_(X) contained in exhaust gas by oneof the absorption and the adsorption or by both the absorption and theadsorption when an air-fuel ratio of the exhaust gas flowing into thecatalyst is lean and reducing and purifying the occluded NO_(X) with areducing component contained in the exhaust gas when an air-fuel ratioof the exhaust gas is a stoichiometric air-fuel ratio or a rich air-fuelratio, the exhaust gas purifying device for an internal combustionengine further comprising: NO_(X) occluding and reducing catalystsarranged in series with each other on the upstream side and thedownstream side of the exhaust gas passage of the internal combustionengine; and an H₂ sensor, which is arranged in series to each other inthe exhaust gas passage between the upstream side NO_(X) occluding andreducing catalyst and the downstream side NO_(X) occluding and reducingcatalyst, for detecting a hydrogen component concentration in theexhaust gas, wherein, when the NO_(X) occluding and reducing catalyst isto reduce and purify NO_(X) occluded during a lean air-fuel ratiooperation of the engine, at the time of executing a regeneratingoperation in which the exhaust gas of a rich air-fuel ratio or astoichiometric air-fuel ratio is supplied to the NO_(X) occluding andreducing catalyst for a predetermined period of time, according to thehydrogen component concentration in the exhaust gas detected by the H₂sensor, an air-fuel ratio of the exhaust gas flowing into the upstreamside NO_(X) occluding and reducing catalyst is controlled.

That is, in the invention described in claim 7, two NO_(X) occluding andreducing catalysts are arranged in series in the exhaust gas passage,and the H₂ sensor is disposed between the upstream side NO_(X) occludingand reducing catalyst and the downstream side NO_(X) occluding andreducing catalyst.

In the so-called tandem type catalyst in which two NO_(X) occluding andreducing catalysts are arranged in series, an amount of the occludedNO_(X) in the lean air-fuel operation of the upstream side (the frontstage) NO_(X) occluding and reducing catalyst and that of the downstreamside (the rear stage) NO_(X) occluding and reducing catalyst aredifferent from each other. Further, a state of the progress of theregeneration at the time of the regenerating operation of the upstreamside (the front stage) NO_(X) occluding and reducing catalyst and thatof the downstream side (the rear stage) NO_(X) occluding and reducingcatalyst are different from each other. Therefore, when the H₂ sensor isdisposed on the downstream side of the rear stage NO_(X) occluding andreducing catalyst, if the regenerating operation is controlled based onthe hydrogen component concentration detected by the thus arranged H₂sensor, problems may occur.

In the present invention, as the H₂ sensor is arranged between the frontstage NO_(X) occluding and reducing catalyst and the rear stage NO_(X)occluding and reducing catalyst, and as the regenerating operation iscontrolled based on the hydrogen component concentration contained inthe exhaust gas detected by the thus arranged H₂ sensor, the NO_(X)occluding and reducing catalyst can be appropriately regenerated.

According to the invention described in claim 8, there is provided anexhaust gas purifying device for an internal combustion engine accordingto claim 7, wherein an NO_(X) occlusion capacity of the upstream sideNO_(X) occluding and reducing catalyst is larger than that of thedownstream side NO_(X) occluding and reducing catalyst, an O₂ storagecapacity of the upstream side NO_(X) occluding and reducing catalyst issmaller than that of the downstream side NO_(X) occluding and reducingcatalyst, and an amount of platinum components carried on the upstreamside NO_(X) occluding and reducing catalyst is larger than that carriedon the downstream side NO_(X) occluding and reducing catalyst.

That is, according to the invention described in claim 8, an NO_(X)occlusion capacity of the upstream side NO_(X) occluding and reducingcatalyst is larger than that of the downstream side NO_(X) occluding andreducing catalyst, while an O₂ storage capacity of the upstream sideNO_(X) occluding and reducing catalyst is smaller than that of thedownstream side NO_(X) occluding and reducing catalyst. Further, anamount of platinum components carried on the upstream side NO_(X)occluding and reducing catalyst is larger than that of the downstreamside NO_(X) occluding and reducing catalyst.

Further, when an amount of supporting Pt of the upstream side NO_(X)occluding and reducing catalyst is increased, most of NO contained inthe exhaust gas is oxidized on the catalyst and changed into NO₂.Therefore, in addition to the setting in which an amount of the occludedNO_(X) per unit volume on the upstream side is set large, the upstreamside NO_(X) occluding and reducing catalyst can effectively occlude andreduce and purify NO_(X).

In the case where two NO_(X) occluding and reducing catalysts arearranged in series to each other, almost all of the occlusion andreducing purification is executed by the upstream side NO_(X) occludingand reducing catalyst. Therefore, by setting NO_(X) occlusion capacity(the maximum amount of NO_(X) can be occluded in the NO_(X) occludingand reducing catalyst) of the upstream side NO_(X) occluding andreducing catalyst at large value, an amount of NO_(X) capable of beingtreated by the upstream side NO_(X) occluding and reducing catalyst canbe increased. Further, by decreasing a storage capacity of storing O₂ inthe upstream side NO_(X) occluding and reducing catalyst, hydrogencomponents and HC and CO components contained in the exhaust gas are notreacted with oxygen occluded in the catalyst in the upstream side NO_(X)occluding and reducing catalyst during the regenerating operation, andmost of HC and CO components in the exhaust gas are used for reducingNO_(X).

According to the invention described in claim 9, there is provided anexhaust gas purifying device for an internal combustion engine accordingto claim 7 or 8, wherein a time for terminating the regeneratingoperation is judged according to the hydrogen component concentration inthe exhaust gas detected by the H₂ sensor at the time of executing theregenerating operation.

According to the invention described in claim 9, the timing at which theregenerating operation is to be terminated is judged according to thehydrogen component concentration in the exhaust gas detected by the H₂sensor disposed between the front stage and the rear stage NO_(X)occluding and reducing catalyst.

In the tandem type NO_(X) occluding and reducing catalyst, at the timeof operation of a lean air-fuel ratio, most of NO_(X) contained in theexhaust gas is occluded in the front stage NO_(X) occluding and reducingcatalyst. Therefore, the amount of NO_(X) occluded in the front stageNO_(X) occluding and reducing catalyst is remarkably larger than theamount of NO_(X) occluded in the rear stage NO_(X) occluding andreducing catalyst. For the above reasons, in the tandem type NO_(X)occluding and reducing catalyst, it is important to sufficientlyregenerate the front stage NO_(X) occluding and reducing catalyst.

In the tandem type NO_(X) occluding and reducing catalyst, it is usualthat the rear stage NO_(X) occluding and reducing catalyst is given arelatively large O₂ storage capacity so that the rear stage NO_(X)occluding and reducing catalyst can be given a function of the three waycatalyst. The O₂ storage capacity is a capacity that the NO_(X)occluding and reducing catalyst occludes oxygen when the air-fuel ratioof the exhaust gas is lean and desorbs occluded oxygen when the air-fuelratio of the exhaust gas becomes rich. Therefore, even if a relativelylarge amount of H₂ components flows into the rear stage NO_(X) occludingand reducing catalyst after the regeneration of the front stage NO_(X)occluding and reducing catalyst is completed, the hydrogen componentsare oxidized by oxygen desorbed from the rear stage NO_(X) occluding andreducing catalyst. Thus, no hydrogen components are detected in theexhaust gas at the outlet of the rear stage NO_(X) occluding andreducing catalyst.

Therefore, when the hydrogen sensor is arranged at the outlet of therear stage NO_(X) occluding and reducing catalyst, it is difficult toaccurately judge the time at which the regenerating operation is to beterminated. That is, although the NO_(X) occlusion capacity of thetandem type NO_(X) occluding and reducing catalyst as a whole isregenerated when the regeneration of the front stage NO_(X) occludingand reducing catalyst is completed, no hydrogen components are detectedin the exhaust gas on the rear stage downstream side. Accordingly, thetime at which the regenerating operation is to be terminated can not beaccurately judged.

According to the present invention, as the H₂ sensor is arranged betweenthe front stage catalyst and the rear stage catalyst of the tandem typeNO_(X) occluding and reducing catalyst and the time at which theregenerating operation is to be terminated is judged based on the outputof the H₂ sensor, the tandem type NO_(X) occluding and reducing catalystcan be appropriately regenerated.

According to the invention described in claim 10, there is provided anexhaust gas purifying device for an internal combustion engine accordingto claim 7 or 8, wherein the device further executes a poisoningregeneration treatment in order to desorb sulfur oxide occluded in theNO_(X) occluding and reducing catalyst together with NO_(X) from theNO_(X) occluding and reducing catalyst by making the air-fuel ratio ofthe exhaust gas flowing into the NO_(X) occluding and reducing catalystto be a rich air-fuel ratio and, at the same time, raising thetemperature thereof and wherein an air-fuel ratio of the exhaust gasflowing into the upstream side NO_(X) occluding and reducing catalyst iscontrolled according to the hydrogen component concentration in theexhaust gas detected by the H₂ sensor in the process of executing thepoisoning regeneration treatment.

That is, according to the invention described in claim 10, the poisoningregeneration treatment control is executed according to an output of theH₂ sensor arranged between the front stage and the rear stage NO_(X)occluding and reducing catalyst.

When the air-fuel ratio is lean, the NO_(X) occluding and reducingcatalyst occludes SO_(X) in the exhaust gas in the same manner as thatof NO_(X). However, as the affinity of SO_(X) with the occlusionmaterial is strong, once SO_(X) is occluded in the NO_(X) occluding andreducing catalyst, it is difficult for SO_(X) to be desorbed from theNO_(X) occluding and reducing catalyst by a mere rich spike operationconducted for the regeneration of the NO_(X) occluding and reducingcatalyst. Therefore, once SO_(X) is occluded in the NO_(X) occluding andreducing catalyst, SO_(X) is gradually accumulated in the catalyst.

Therefore, when an amount of SO_(X) occluded in the NO_(X) occluding andreducing catalyst is increased, an amount of NO_(X) capable of beingoccluded in the NO_(X) occluding and reducing catalyst is decreased,that is, the NO_(X) occlusion capacity is decreased. That is, aso-called SO_(X) poisoning is caused.

In order to solve the problem of SO_(X) poisoning, it is necessary toconduct a poisoning regeneration treatment in which, while the air-fuelratio of the exhaust gas flowing into the NO_(X) occluding and reducingcatalyst is being maintained at a rich air-fuel ratio, a temperature ofthe NO_(X) occluding and reducing catalyst is raised. Even in thepoisoning regeneration treatment, when hydrogen components are suppliedto the NO_(X) occluding and reducing catalyst, an SO_(X) desorbing rate,at which SO_(X) is desorbed from the NO_(X) occluding and reducingcatalyst, is increased, and the poisoning regeneration treatment can becompleted in a short period of time.

In this case, in order to appropriately conduct the poisoningregeneration treatment, for example, it is necessary to appropriatelyjudge the timing for terminating the poisoning regeneration treatment inthe same manner as that of the regenerating operation of the NO_(X)occluding and reducing catalyst. However, in the tandem type NO_(X)occluding and reducing catalyst, there are some peculiar conditions inwhich, for example, an amount of SO_(X) occluded in the front stageNO_(X) occluding and reducing catalyst is different from an amount ofSO_(X) occluded in the rear stage NO_(X) occluding and reducingcatalyst. Therefore, if the poisoning regeneration treatment isconducted according to the output of the H2 sensor disposed on thedownstream side of the NO_(X) occluding and reducing catalyst, it isdifficult to appropriately conduct the poisoning regeneration treatment.

According to the present invention, the poisoning regeneration treatmentcontrol is executed according to an output of the H2 sensor arrangedbetween the front stage and the rear stage NO_(X) occluding and reducingcatalyst of the tandem type. Therefore, in addition to the regenerationof the NO_(X) occluding and reducing catalyst, the poisoningregeneration treatment can be appropriately executed.

According to the invention described in claim 11 there is provided anexhaust gas purifying device for an internal combustion engine accordingto claim 10, wherein a time for terminating the poisoning regenerationtreatment is judged according to the hydrogen component concentration inthe exhaust gas detected by the H₂ sensor at the time of executing thepoisoning regeneration treatment.

As hydrogen has a very strong reducing capability, when hydrogencomponents are contained in the exhaust gas flowing into the NO_(X)occluding and reducing catalyst at the time of the poisoningregeneration treatment, the hydrogen components are immediately reactedwith SO_(X) desorbed from the NO_(X) occluding and reducing catalyst.Therefore, in the same manner as that of the rich spike operation, evenat the time of the poisoning regeneration treatment, while SO_(X) isbeing desorbed, the hydrogen components contained in the exhaust gas areconsumed for reducing SO_(X). Therefore, no hydrogen flows out into theexhaust gas on the downstream side of the NO_(X) occluding and reducingcatalyst.

Accordingly, in the same manner as that of the judgment of thecompletion of the regeneration of the NO_(X) occluding and reducingcatalyst, even at the time of the poisoning regeneration treatment, thecompletion of the desorption of SO_(X) can be judged according to anoutput of the H₂ sensor disposed on the downstream side of the NO_(X)occluding and reducing catalyst.

However, in this case, in the tandem type NO_(X) occluding and reducingcatalyst, most of SO_(X) contained in the exhaust gas is occluded in thefront stage NO_(X) occluding and reducing catalyst. Therefore, it isimportant that the front stage NO_(X) occluding and reducing catalyst issufficiently regenerated from the SO_(X) poisoning.

In the tandem type NO_(X) occluding and reducing catalyst, the rearstage NO_(X) occluding and reducing catalyst carries a relatively largeamount of ceria (Ce) component in order to enhance the O₂ storagecapacity thereof. Therefore, SO_(X) contained in the exhaust gas isbonded to ceria in the rear stage NO_(X) occluding and reducingcatalyst, and sulfate is formed. In this case, a bonding strength ofceria and SO_(X) is not so strong and SO_(X) can be relatively easilydesorbed from the rear stage NO_(X) occluding and reducing catalyst.

Therefore, in the poisoning regeneration treatment of the tandem typeNO_(X) occluding and reducing catalyst, at a point of time when thepoisoning regeneration treatment of the front stage NO_(X) occluding andreducing catalyst is terminated, all SO_(X) has already been desorbedfrom the rear stage NO_(X) occluding and reducing catalyst. Accordingly,the poisoning regeneration treatment can be performed more effectivelyby judging the timing for terminating the poisoning regenerationtreatment based on the state of desorption of SO_(X) from the frontstage NO_(X) occluding and reducing catalyst.

According to the present invention, at the time of the poisoningregeneration treatment, when hydrogen is detected in the exhaust gas bythe H₂ sensor arranged at a position between the front stage and therear stage NO_(X) occluding and reducing catalyst, the poisoningregeneration treatment is terminated. Therefore, not only theregeneration of the NO_(X) occluding and reducing catalyst but also theregeneration from SO_(X) poisoning can be appropriately executed.

According to the invention described in claim 12, there is provided anexhaust gas purifying device for an internal combustion engine accordingto claim 8, wherein a degree of deterioration of the upstream sideNO_(X) occluding and reducing catalyst is judged according to a periodof time from the start of the regenerating operation to the end of theregenerating operation which is judged according to the hydrogencomponent concentration in the exhaust gas detected by the H₂ sensor.

That is, according to the invention described in claim 12, a degree ofdeterioration of the front stage NO_(X) occluding and reducing catalystof the tandem type NO_(X) occluding and reducing catalyst is judged bythe output of the H₂ sensor arranged between the front stage and therear stage.

In the tandem type NO_(X) occluding and reducing catalyst, the NO_(X)occlusion capacity of the front stage NO_(X) occluding and reducingcatalyst is more important than the NO_(X) occlusion capacity of therear stage NO_(X) occluding and reducing catalyst. On the other hand,the deterioration of the tandem type NO_(X) occluding and reducingcatalyst advances from the front stage NO_(X) occluding and reducingcatalyst. Therefore, in order to accurately judge a degree of thedeterioration of the tandem type NO_(X) occluding and reducing catalyst,the hydrogen sensor must be arranged not on the downstream side of therear stage NO_(X) occluding and reducing catalyst but between the frontand the rear stage NO_(X) occluding and reducing catalyst. In thepresent invention, the H₂ sensor is arranged at a position between thefront stage and the rear stage NO_(X) occluding and reducing catalyst.Therefore, according to the present invention, the tandem type NO_(X)occluding and reducing catalyst can be appropriately regenerated. At thesame time, it is possible to accurately judge a degree of thedeterioration of the front stage NO_(X) occluding and reducing catalyst.

From the foregoing explanation, it will be understood that, according tothe invention described in each claim, by controlling the regeneratingoperation of the NO_(X) occluding and reducing catalyst based on aconcentration of hydrogen components contained in exhaust gas detectedby H₂ sensor, the NO_(X) occluding and reducing catalyst can beappropriately regenerated.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is an arrangement view showing an outline of an embodiment inwhich the present invention is applied to an internal combustion enginefor automobile use,

FIG. 2 is a view schematically showing a relation between a hydrogencomponent concentration in exhaust gas and an air-fuel ratio of exhaustgas,

FIG. 3 is a flowchart for explaining an operation of regenerating anNO_(X) occluding and reducing catalyst in the first embodiment of thepresent invention,

FIG. 4 is a flowchart for explaining an operation of regenerating anNO_(X) occluding and reducing catalyst in the second embodiment of thepresent invention,

FIG. 5 is a flowchart for explaining an operation of regenerating anNO_(X) occluding and reducing catalyst in the third embodiment of thepresent invention,

FIG. 6 is a view for explaining a setting of a hydrogen concentrationtarget value at the time of NO_(X) occluding and reducing catalystregenerating operation in the fourth embodiment of the presentinvention,

FIG. 7 is a flowchart for explaining a deterioration judgment operationof an NO_(X) occluding and reducing catalyst in the fifth embodiment ofthe present invention,

FIG. 8 is a view similar to FIG. 1, for explaining a structure of thesixth embodiment of the present invention and

FIG. 9 is a flowchart for explaining a poisoning regeneration treatmentof an NO_(X) occluding and reducing catalyst in the seventh embodimentof the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

By referring to the accompanying drawings, an embodiment of the presentinvention will be explained below.

FIG. 1 is an arrangement view showing an outline of an embodiment inwhich the present invention is applied to an internal combustion enginefor automobile use.

In FIG. 1, reference numeral 1 is an internal combustion engine for anautomobile. In this embodiment, the engine 1 is a 4-cylinder gasolineengine having 4 cylinders from #1 to #4. In the cylinders from #1 to #4,fuel injection valves 111 to 114 for injecting fuel are arranged ininlet ports of the respective cylinders. The engine 1 is a lean-burnengine which is capable of operating in a wide air-fuel ratio range froma rich air-fuel ratio to a lean air-fuel ratio and, in this embodiment,is operated at a lean air-fuel ratio in a greater part of the operationregion.

In this embodiment, the cylinders #1 to #4 are divided into two cylindergroups. In this case, each cylinder group is composed of two cylinders,the ignition timings of which are not adjacent to each other. Forexample, in the embodiment shown in FIG. 1, the ignition order ofigniting the cylinders is 1-3-4-2. Therefore, the cylinders #1 and #4compose a cylinder group, and the cylinders #2 and #3 compose a cylindergroup. An exhaust port of each cylinder is connected to an exhaustmanifold of each cylinder. The exhaust manifold of each cylinder isconnected to an exhaust passage of each cylinder group.

In FIG. 1, reference numeral 21 a is an exhaust manifold for connectingexhaust ports of a group of cylinders composed of the cylinders #1 and#4 to an individual exhaust passage 2 a, and reference numeral 21 b isan exhaust manifold for connecting exhaust ports of a group of cylinderscomposed of the cylinders #2 and #3 to an individual exhaust passage 2b. In this embodiment, in the individual exhaust passages 2 a, 2 b,start catalysts 5 a, 5 b composed of three way catalysts arerespectively arranged. The individual exhaust passages 2 a, 2 b join toa common exhaust passage 2 on the downstream side of the startcatalysts.

In the common exhaust passage 2, a converter 70 is arranged in which theNO_(X) occluding and reducing catalyst 7 described later is accommodatedin a casing.

In FIG. 1, reference numerals 31, 33 are an upstream side H₂ sensor anda downstream side H₂ sensor which are respectively disposed on an inletside and an outlet side of the converter 70 arranged in the exhaustpassage 2. These H₂ sensors detect a concentration of hydrogen (H₂)components contained in the exhaust gas.

In FIG. 1, reference numeral 30 is an electronic control unit (ECU) ofthe engine 1. In this embodiment, ECU 30 is a known type microcomputerincluding RAM, ROM and CPU. ECU 30 conducts basic control of the enginesuch as ignition timing control, fuel injection control and so forth.

In addition to the basic control stated above, ECU 30 in this embodimentconducts a regenerating operation in which every time an amount ofNO_(X) occluded in the NO_(X) occluding and reducing catalyst 7 isincreased to a predetermined amount, amounts of fuel injection of theinjection valves 111 to 114 are increased so as to operate the enginefor a short period of time at a rich air-fuel ratio or a stoichiometricair-fuel ratio. In this way, the NO_(X) occluding and reducing catalyst7 desorbs the occluded NO_(X) so that the exhaust gas can be reduced andpurified.

Further, in the present embodiment, ECU 30 controls an air-fuel ratio ofthe exhaust gas at the time of regenerating operation according to theconcentration of hydrogen components in the exhaust gas at the inlet orthe outlet of the NO_(X) occluding and reducing catalyst 7 detected bythe H₂ sensors 31, 33 at the time of conducting the above regeneratingoperation.

In order to conduct control as described above, the following signals,which are parameters expressing a state of operation of the engine, areinputted into input ports of ECU 30. The signals to be inputted are: asignal corresponding to the inlet pressure of the engine sent from theinlet pressure sensor 41 provided in the engine inlet manifold notshown; a signal corresponding to the engine speed sent from the enginespeed sensor 43 arranged close to the engine crank shaft not shown; anda signal expressing an amount of the depression of the brake pedal (adegree of opening of the acceleration pedal) sent from the accelerationopening degree sensor 45 arranged close to the acceleration pedal notshown of the engine 1. Further, concentrations of H₂ contained in theexhaust gas at the inlet and the outlet of the NO_(X) occluding andreducing catalyst 7 sent from the H₂ sensors 31, 33 are inputted.

Output ports of ECU 30 are connected to the fuel injection valves 111 to114 of the cylinders via a fuel injection circuit not shown in order tocontrol an amount of fuel injection into each cylinder and a fuelinjection timing.

Next, the NO_(X) occluding and reducing catalyst 7 of this embodimentwill be explained below.

The NO_(X) occluding and reducing catalyst 7 of the present embodimentis composed as follows. For example, a catalyst substrate made ofcordierite, which is formed into a honeycomb shape, is used. On asurface of this catalyst substrate, an aluminum coating is provided. Onthis aluminum layer, one component selected from a group of alkali metalsuch as potassium K, sodium Na, lithium Li and cesium Cs, alkali earthmetal such as barium Ba and calcium Ca, and rare earth metal such aslanthanum La, cerium Ce and ytrium Y, and one component selected fromprecious metal such as platinum Pt are carried. In the case where anair-fuel ratio of the exhaust gas flowing into the NO_(X) occluding andreducing catalyst is lean, the NO_(X) occluding and reducing catalystabsorbs NO_(X) (NO₂, NO) in the exhaust gas in the form of nitric acidions NO₃ ⁻. When the oxygen concentration of the exhaust gas is lowered,the NO_(X) occluding and reducing catalyst discharges occluded NO_(X),in other words, the NO_(X) occluding and reducing catalyst conductsoccluding and desorbing action of NO_(X) in accordance with the air-fuelratio of the exhaust gas.

That is, in the case where the engine 1 is operated at a lean air-fuelratio and the exhaust gas flowing into the NO_(X) occluding and reducingcatalyst is at a lean air-fuel ratio, NO_(X) (NO) contained in theexhaust gas is oxidized, for example, on platinum Pt and changed intoNO₂ and further oxidized to form nitric acid ions. In the case where,for example, BaO is used as the occlusion material, these nitric acidions are absorbed in the occlusion material and bonded to barium oxideBaO and diffused in the occlusion material in the form of nitric acidions NO₃ ⁻. Therefore, in the lean atmosphere, NO_(X) contained in theexhaust gas is occluded in the NO_(X) occluding and reducing catalyst inthe form of nitrate.

In the case where the oxygen concentration in the exhaust gas flowinginto the NO_(X) occluding and reducing catalyst is decreased, that is,in the case where the air-fuel ratio of the exhaust gas becomes astoichiometric air-fuel ratio or a rich air-fuel ratio, an amount ofgeneration of nitric ions on platinum Pt is decreased. Therefore, thereaction proceeds in a reverse direction, and the nitric acid ions NO₃ ⁻in the occlusion material are desorbed from the occlusion material inthe form of NO₂. In this case, when components functioning as a reducingagent such as CO or H₂ exist in the exhaust gas or in the case where HCcomponents exist in the exhaust gas, NO₂ is reduced on platinum Pt bythese components.

In the atmosphere of a lean air-fuel ratio, the NO_(X) occluding andreducing catalyst 7 occludes NO_(X) contained in the exhaust gas in theocclusion material (for example, BaO) in the form of nitric ions by theaforementioned mechanism. Therefore, as the nitric acid ionconcentration in the occlusion material is increased, it becomesdifficult for new nitric acid ions to be absorbed in the occlusionmaterial, and a ratio of purifying NO_(X) in the exhaust gas is lowered.When an amount of NO_(X) occluded in the NO_(X) occluding and reducingcatalyst reaches an upper limit, that is, the nitric acid ionconcentration in the occlusion material is increased and reaches thesaturated concentration, it becomes completely difficult for the NO_(X)occluding and reducing catalyst to occlude NO_(X) contained in theexhaust gas.

In the present embodiment, ECU 30 estimates an amount of NO_(X)generated from the engine 1 per unit time based on the parameters whichrepresent the engine operating condition, such as an engine inletpressure, an engine speed and an acceleration pedal opening degree,using relationships established by experiment in advance. Apredetermined percentages of the amount of NO_(X) generated by theengine per unit time is considered to correspond the amount of NO_(X)occluded in the NO_(X) occluding and reducing catalyst per unit time.Therefore, the amount of NO_(X) occluded by the NO_(X) occluding andreducing catalyst 7 can be calculated by integrating the predeterminedpercentage of the amount of NO_(X) generated by the engine per unittime. Thus the integrated value, which is referred to as an NO_(X)counter, corresponds to an amount of NO_(X) which has been occluded inthe NO_(X) occluding and reducing catalyst 7.

Further, ECU 30 regenerates the NO_(X) occluding and reducing catalyst 7by carrying out a rich spike operation in which the exhaust gas of arich air-fuel ratio is supplied to the NO_(X) occluding and reducingcatalyst 7 by operating the engine 1 in a short period at a richair-fuel ratio each time this NO_(X) counter reaches a predeterminedvalue. Due to the foregoing, the NO_(X) occluding and reducing catalyst7 always occludes NO_(X) under the condition that an amount of occludedNO_(X) is relatively low. Accordingly, it is possible to maintain theNO_(X) purifying ratio of the NO_(X) occluding and reducing catalyst tobe high.

In this connection, instead of estimating the NO_(X) occlusion amount ofthe NO_(X) occluding and reducing catalyst 7 with the NO_(X) counter asdescribed above, the timing for executing the rich-spike can be judgedby disposing an NO_(X) sensor for detecting the NO_(X) concentration inthe exhaust gas on the downstream side of the NO_(X) occluding andreducing catalyst 7.

When the NO_(X) occlusion amount of the NO_(X) occluding and reducingcatalyst 7 is increased, the NO_(X) purifying capacity of the NO_(X)occluding and reducing catalyst 7 is lowered, and a part of NO_(X) inthe exhaust gas passes through the NO_(X) occluding and reducingcatalyst 7 without being occluded therein. Therefore, by disposing aNO_(X) sensor on the downstream side of the NO_(X) occluding andreducing catalyst 7, the rich-spike operation may be executed when theconcentration of NO_(X) in the exhaust gas detected by the NO_(X) sensorincreases and reaches a predetermined value (i.e., when the amount ofNO_(X) occluded in the NO_(X) occluding and reducing catalyst 7increased).

(1) FIRST EMBODIMENT

Referring to FIGS. 2 and 3, the first embodiment of the presentinvention will be explained below.

As described above, in the present embodiment, each time the NO_(X)occlusion amount of the NO_(X) occluding and reducing catalyst 7 reachesa predetermined amount, the rich spike operation is conducted so as toregenerate the NO_(X) occluding and reducing catalyst 7.

In order to appropriately regenerate the NO_(X) occluding and reducingcatalyst, it is necessary that the rich spike operation is carried outuntil the regeneration of the NO_(X) occluding and reducing catalyst iscompleted and that the engine operation air-fuel ratio is returned to alean air-fuel ratio after the regeneration has been completed.

Conventionally, the completion of the regeneration of the NO_(X)occluding and reducing catalyst was judged based on the output of an O₂sensor disposed on the downstream side of the NO_(X) occluding andreducing catalyst as described in Japanese Patent Publication No.2692380, and when the air-fuel ratio on the catalyst downstream sidedetected by the O₂ sensor is changed from a stoichiometric air-fuelratio to a lean air-fuel ratio in the process of rich spike operation,it is judged that the regeneration of the NO_(X) occluding and reducingcatalyst is completed. The air-fuel ratio of the exhaust gas is returnedto the lean air-fuel ratio when the regeneration of the NO_(X) occludingand reducing catalyst is completed.

However, as described before, when an amount of HC components containedin the exhaust gas is large, HC components are attached onto a surfaceof the catalyst component of the NO_(X) occluding and reducing catalyst,and covering, which deteriorates the catalyst function, is caused. Whencovering is caused, a portion of the HC components contained in theexhaust gas flowing into the NO_(X) occluding and reducing catalyst passthrough the catalyst without reacting with NO_(X). Therefore, on thedownstream side of the NO_(X) occluding and reducing catalyst, even whenregeneration of the NO_(X) occluding and reducing catalyst has not beencompleted yet, the air-fuel ratio becomes rich.

Therefore, when the completion of regeneration of the NO_(X) occludingand reducing catalyst is judged according to an output of the O₂ sensordisposed on the downstream side of the NO_(X) occluding and reducingcatalyst, even if the regeneration of the catalyst has not beencompleted yet, the rich spike operation is terminated in some cases.Therefore, the regeneration of the NO_(X) occluding and reducingcatalyst cannot be sufficiently executed.

Therefore, in the present embodiment, the above problems are solved byterminating the rich-spike operation when it is judged that theregeneration of the NO_(X) occluding and reducing catalyst is completedusing the output of the H₂ sensor disposed on the downstream side of theNO_(X) occluding and reducing catalyst.

For example, the water gas shift reaction (CO+H₂O→CO₂+H₂) or steamreforming (HC+H₂O→CO₂+H₂) is caused from HC, CO and H₂O which aregenerated at the time of combustion when the air-fuel ratio of theexhaust gas of the engine becomes rich, and hydrogen is generated. Inthe case of a usual internal combustion engine, H₂ is generated by theabove reaction during a rich air-fuel ratio operation, however, thesereactions are further facilitated by a three-way catalyst. Therefore, inthe internal combustion engine having the three-way catalyst such as thestart catalysts 5 a, 5 b in the exhaust gas passage on the upstream sideof the NO_(X) occluding and reducing catalyst, a relatively large amountof hydrogen is contained in the exhaust gas during the rich-spikeoperation for regenerating the NO_(X) occluding and reducing catalyst.

Other than the three way catalyst, if a hydrogen generation catalystwhich effectively generates the water gas shift reaction or the steamreforming is disposed in the exhaust gas passage, it is also possible togenerate hydrogen during the operation of the engine at a rich air-fuelratio.

FIG. 2 is a graph showing a relation between the air-fuel ratio of theexhaust gas and the amount of generation of H₂. FIG. 2 shows a relationbetween the amount of generation of H₂ in the start catalysts 5 a, 5 band the air-fuel ratio of the exhaust gas. As shown in FIG. 2, when theair-fuel ratio is lean, the amount of generation of H₂ at the three waycatalyst is zero. However, when the air-fuel ratio exceeds thestoichiometric air-fuel ratio and becomes rich, that is, when theair-fuel ratio is decreased, the amount of generation of H₂ at the threeway catalyst is substantially linearly increased. Although the actualamounts of generated H₂ are different from each other, the amount ofgeneration of H₂ in the engine combustion chamber and the amount ofgeneration of H₂ in the case of using H₂ generation catalyst instead ofthe three way catalyst are substantially linearly increased when theair-fuel ratio is decreased.

The hydrogen component generated at the time of the rich spike operationas described above has a very strong reducing power. Therefore, the thusgenerated hydrogen component can directly react with NO_(X) componentwithout using a catalyst component such as Pt. Therefore, even when arelatively large amount of HC components are contained in the exhaustgas and covering is caused and the catalytic function is deteriorated,the hydrogen components in the exhaust gas excellently react with NO_(X)which has desorbed from the NO_(X) occluding and reducing catalyst.

Therefore, as long as NO_(X) is desorbed from the NO_(X) occluding andreducing catalyst, the hydrogen components contained in the exhaust gasare consumed by reacting with NO_(X) on the NO_(X) occluding andreducing catalyst. Therefore, the hydrogen components contained in theexhaust gas do not flow out onto the downstream side of the NO_(X)occluding and reducing catalyst.

Accordingly, in the case where the hydrogen components contained in theexhaust gas are detected by H₂ sensor 33 disposed on the downstream sideof the NO_(X) occluding and reducing catalyst while the rich spikeoperation is being executed, it can be judged that the desorption(regeneration) of NO_(X) from the NO_(X) occluding and reducing catalysthas been completed.

In this connection, concerning the H₂ sensor 33 (31) of this embodiment,for example, it is possible to use a Pd/Ni alloy sensor which isespecially responsive to hydrogen.

This type H₂ sensor is manufactured by KK Toyoda Micro Systems and puton the market under the trademark of “H2 scan”. However, the sensor tobe used in the present embodiment is not limited to the above specificsensor. As long as the sensor can continuously monitor the concentrationof H₂ contained in the exhaust gas with a quick response, any sensor canbe used in the present embodiment.

Next, the regenerating operation of the NO_(X) occluding and reducingcatalyst of this embodiment will be explained below.

FIG. 3 is a flowchart for specifically explaining the regeneratingoperation of the NO_(X) occluding and reducing catalyst of thisembodiment. This operation is conducted as a routine executed at regularintervals by ECU 30.

In the operation shown in FIG. 3, in step 301, it is judged whether ornot the value of the regenerating operation execution flag X is setat 1. In the flag X, the value is set at 1 when the NO_(X) occlusionamount of the NO_(X) occluding and reducing catalyst 7 is increased to apredetermined value in the operation (not shown) separately executed byECU 30. As described before, in this embodiment, by the NO_(X) occlusionamount estimating operation (not shown) separately executed, ECU 30calculates the aforementioned NO_(X) counter value, which expresses theNO_(X) occlusion amount of the NO_(X) occluding and reducing catalyst,at regular intervals. When this NO_(X) counter value reaches apredetermined value, the value of the regenerating operation executingflag x is set at 1.

In the case of X≠1 in step 301, an amount of NO_(X) occlusion of theNO_(X) occluding and reducing catalyst 7 has not reached a predeterminedvalue. Therefore, it is unnecessary to regenerate the NO_(X) occludingand reducing catalyst 7. Accordingly, this operation is immediatelyterminated. In this case, the rich spike operation is not carried outand the engine 1 continues to conduct the lean air-fuel ratio operation.

In the case of X=1 in step 301, the program proceeds to step 303 and therich spike operation (RS) is carried out. In the rich spike operation,the engine 1 operates at a predetermined rich air-fuel ratio. As aresult, hydrogen is generated in the engine 1 and the start catalysts 5a, 5 b. Therefore, the exhaust gas of a rich air-fuel ratio containinghydrogen components flows into the NO_(X) occluding and reducingcatalyst 7.

Next, in step 305, the hydrogen component concentration HRR in theexhaust gas on the downstream side of the NO_(X) occluding and reducingcatalyst is read in from the H₂ sensor on the downstream side of theNO_(X) occluding and reducing catalyst 7. In step 307, it is judgedwhether or not the hydrogen component concentration HRR is not less thanthe predetermined value α. In this case, α is a judgment value forpreventing the occurrence of an erroneous judgment and set at a positivevalue close to zero.

As described before, while NO_(X) is being desorbed from the NO_(X)occluding and reducing catalyst 7 at the time of carrying out RSoperation, the hydrogen components contained in the exhaust gas isreacted with NO_(X) and consumed. Therefore, the hydrogen componentconcentration HRR, which is detected by the downstream side H₂ sensor33, becomes zero. When the desorption of NO_(X) is completed, that is,when the regeneration of the NO_(X) occluding and reducing catalyst iscompleted, hydrogen is detected by the downstream side H₂ sensor 33.

In this embodiment, when the hydrogen component concentration exceedsthe judgment value α in step 307, it is judged that the regeneration ofthe NO_(X) occluding and reducing catalyst 7 is completed, and theprogram proceeds to step 309 and the value of the regenerating operationexecuting flag is set at zero. When the value of the regeneratingoperation executing flag is set at zero, the value of the NO_(X) counteris returned to zero in the aforementioned NO_(X) counter calculationoperation.

In the next execution of this operation, the operation is immediatelyterminated after step 301, and the rich spike operation is terminatedand the air-fuel ratio of the exhaust gas is returned to the leanair-fuel ratio.

On the other hand, in the case of HRR<α in step 307, it is judged thathydrogen components do not flow out onto the downstream side of theNO_(X) occluding and reducing catalyst 7 and the regeneration of theNO_(X) occluding and reducing catalyst 7 is not completed. Therefore,the execution of this operation is terminated while the value of theflag X is being maintained at 1. Due to the foregoing, the rich spikeoperation of step 303 is continued even in the next execution of thisoperation.

In this embodiment, according to an output of the H₂ sensor 33 disposedon the downstream side of the NO_(X) occluding and reducing catalyst 7,the completion of the regeneration of the NO_(X) occluding and reducingcatalyst 7 is accurately judged and the rich spike operation isterminated, that is, the air-fuel ratio of the exhaust gas is returnedto the lean air-fuel ratio. Accordingly, the NO_(X) occluding andreducing catalyst 7 can be appropriately regenerated.

In this connection, in the arrangement shown in FIG. 1, the H₂ sensors31, 33 are respectively disposed on the upstream side and the downstreamside of the NO_(X) occluding and reducing catalyst 7. However, ofcourse, the H₂ sensor 33 may be disposed only on the downstream side ofthe NO_(X) occluding and reducing catalyst 7.

(2) SECOND EMBODIMENT

Next, the second embodiment of the present invention will be explainedbelow.

In the first embodiment described above, when hydrogen components aredetected in the exhaust gas by the H₂ sensor 33 disposed on thedownstream side of the NO_(X) occluding and reducing catalyst 7 in theprocess of executing the rich spike operation, the rich spike operationis terminated and the air-fuel ratio of the exhaust gas is immediatelychanged over to a lean air-fuel ratio. However, the present embodimentis different from the first embodiment at the following points. In thepresent embodiment, after the rich spike operation is terminated, theair-fuel ratio of the exhaust gas is maintained at the stoichiometricair-fuel ratio for a predetermined period of time, and then theair-ratio is returned to a lean air-fuel ratio.

As described before, a high-occlusion type NO_(X) occluding and reducingcatalyst, the NO_(X) occlusion capacity of which is enhanced, has beenrecently used. In the high-occlusion type NO_(X) occluding and reducingcatalyst, the affinity of the occlusion material with NO_(X) is high.Therefore after a relatively large amount of NO_(X) has been desorbed atthe initial stage of the rich spike operation, a rate of desorption ofNO_(X) is decreased. Accordingly, in order to completely regenerate theNO_(X) occluding and reducing catalyst, it is necessary to continue theregenerating operation over a long period of time. However, when theengine is operated at a rich air-fuel ratio over a long period of time,a problem is caused that the fuel consumption of the engine isincreased. When the rich air-fuel ratio operation is continued under thecondition that the rate of desorption of NO_(X) is low, HC and COcontained in the exhaust gas do not react with NO_(X) but flow out tothe downstream side of the NO_(X) occluding and reducing catalyst.Therefore, the exhaust emission is deteriorated. Therefore, in thepresent embodiment, after a relatively large amount of NO_(X) has beendesorbed from the NO_(X) occluding and reducing catalyst in the initialstage, the rich spike operation is completed and the engine is operatedat the stoichiometric air-fuel ratio and the NO_(X) occluding andreducing catalyst is completely regenerated over a relatively longperiod of time.

In the present embodiment, the time at which the rich spike operation isterminated (the time at which the air-fuel ratio is changed over to thestoichiometric air-fuel ratio) is judged according to the output of theH₂ sensor 33 disposed on the downstream side of the NO_(X) occluding andreducing catalyst 7.

That is, while the rich spike operation is being carried out, theexhaust gas of a rich air-fuel ratio containing hydrogen componentsflows into the NO_(X) occluding and reducing catalyst. While NO_(X) isbeing desorbed from the NO_(X) occluding and reducing catalyst 7, thehydrogen components contained in the exhaust gas is reacted with NO_(X)and consumed. Therefore, the H₂ sensor 33 disposed on the downstreamside does not detect the hydrogen components.

However, in the case of a high occlusion type NO_(X) occluding andreducing catalyst, when the initial desorption of NO_(X) is finished inthe process of executing the spike rich operation and a rate ofdesorption of NO_(X) is sharply decreased, the amount of NO_(X) is low.Therefore, a portion of hydrogen components contained in the exhaust gasflows out onto the downstream side of the catalyst without reacting withNO_(X).

Therefore, in the present embodiment, when NO_(X) is detected in theexhaust gas by the H₂ sensor 33 disposed on the downstream side at thetime of executing the rich spike operation, it is judged that theinitial desorption of NO_(X) from the NO_(X) occluding and reducingcatalyst 7 is completed, and the air-fuel ratio of the exhaust gas ischanged over to the stoichiometric air-fuel ratio.

FIG. 4 is a flowchart for explaining the regenerating operation of theNO_(X) occluding and reducing catalyst of the present embodimentdescribed above. This operation is conducted by ECU 30 as a routine tobe executed at regular intervals.

In this embodiment, the NO_(X) occluding and reducing catalyst isregenerated by using the stoichiometric air-fuel ratio holding flag Y inaddition to using the rich spike operation executing flag X. The valueof the flag Y is set at 1 together with the value of the flag X when thevalue of the NO_(X) counter reaches a predetermined value in the NO_(X)occlusion amount estimating operation described before.

In FIG. 4, when the operation is started, it is judged in step 401whether or not the value of the stoichiometric air-fuel ratio holdingflag Y is set at 1. As described later the value of the flag Y is set at0 (step 419) after a predetermined period of time has passed (steps 413to 415) from when the value of the flag X is set at 0 (step 411) afterthe completion of the rich spike operation. Therefore, in the case ofY≠1 in step 401, the rich spike operation and the stoichiometricair-fuel ratio holding operation conducted after the rich spikeoperation are surely completed.

Therefore, in the case of Y≠1 in step 401, the operation of step 403 andafter is not carried out and the operation of the present time isimmediately terminated.

In the case of Y=1 in step 401, as the NO_(X) occluding and reducingcatalyst is being regenerated, the program proceeds to step 403 andwhether or not the rich spike operation in the initial stage ofregenerating operation is completed is judged according to the value ofthe flag X. In the case of X=1 (not completed), the rich air-fuel ratiooperation is continued until hydrogen components are detected in theexhaust gas by the H₂ sensor 33 provided on the downstream side of theNO_(X) occluding and reducing catalyst. The operation described in steps403 to 411 is the same as that described in steps 301 to 309 shown inFIG. 3.

On the other hand, in the case of X=1 in step 403, this means thatalthough the rich spike operation has already been completed, thestoichiometric air-fuel ratio holding operation is not terminated yet.Therefore, the program proceeds to step 413 and the value of the counterCT is increased by 1, and the program proceeds to step 415 and it isjudged whether or not the value of the counter CT, which was increased,has reached a predetermined value β. In this case, β is a counter valuecorresponding to a period of time in which the NO_(X) occluding andreducing catalyst should be maintained at the stoichiometric air-fuelratio after the completion of the rich spike operation. That is, β is aperiod of time necessary for completing the regeneration of the NO_(X)occluding and reducing catalyst at the stoichiometric air-fuel ratioafter the desorption of NO_(X) in the initial stage. The value of βdiffers according to the capacity and the type of the NO_(X) occludingand reducing catalyst. Therefore, it is preferable that the value of βis determined by experiment in which an actual NO_(X) occluding andreducing catalyst is used.

In the case of CT<β in step 415, the regeneration of the NO_(X)occluding and reducing catalyst has not been completed yet. Therefore,the program proceeds to step 417 and the engine is operated at thestoichiometric air-fuel ratio. Due to the foregoing, the exhaust gas ofthe stoichiometric air-fuel ratio flows into the NO_(X) occluding andreducing catalyst 7, and the regeneration of the NO_(X) occluding andreducing catalyst 7 is continued without greatly increasing the fuelconsumption of the engine 1.

In the case of CT≧β in step 415, as the regeneration of the NO_(X)occluding and reducing catalyst 7 has already been completed, theprogram proceeds to step 419, and the values of the flag Y and thecounter CT are reset to 0. Due to the foregoing, in the next operation,the operation is terminated immediately after the operation of step 401.Therefore, the regenerating operation of the NO_(X) occluding andreducing catalyst is not conducted and a usual lean air-fuel ratiooperation is conducted.

According to the embodiment shown in FIG. 4, when hydrogen componentscontained in the exhaust gas are detected by the H₂ sensor 33 providedon the downstream side of the NO_(X) occluding and reducing catalyst 7in the process of the regenerating operation, the rich spike operationis terminated and the exhaust gas is held at a predeterminedstoichiometric air-fuel ratio after that. Due to the foregoing, evenwhen a high occlusion type NO_(X) occluding and reducing catalyst isused, it is possible to appropriately regenerate the NO_(X) occludingand reducing catalyst 7.

(3) THIRD EMBODIMENT

Next, the third embodiment will be explained below.

In the first and the second embodiment described above, the time forterminating the regenerating operation of the NO_(X) occluding andreducing catalyst and the time for terminating the rich spike operationare judged by the hydrogen concentration detected by the downstream sideH₂ sensor 33 provided on the downstream side of the NO_(X) occluding andreducing catalyst 7.

On the other hand, in the present embodiment, the air-fuel ratio of theexhaust gas is controlled so that the hydrogen component concentrationdetected by the upstream side H₂ sensor 31 disposed on the upstream sideof the NO_(X) occluding and reducing catalyst becomes a predeterminedvalue during the regenerating operation of the NO_(X) occluding andreducing catalyst 7.

As explained in FIG. 2, the hydrogen component concentration in theexhaust gas changes according to the air-fuel ratio of the exhaust gas.Accordingly, if the air-fuel ratio of the exhaust gas at the time of aregenerating operation of the NO_(X) occluding and reducing catalyst ischanged, the hydrogen component concentration in the exhaust gas is alsochanged and, in some cases, the amount of hydrogen supplied to theNO_(X) occluding and reducing catalyst becomes insufficient. This maycause an insufficient regeneration of the NO_(X) occluding and reducingcatalyst.

In the present embodiment, the air-fuel ratio of the exhaust gas isfeedback controlled based on the output of the upstream side H₂ sensor31 so that the hydrogen component concentration in the exhaust gasflowing into the NO_(X) occluding and reducing catalyst becomes apredetermined value at the time of a regenerating operation of theNO_(X) occluding and reducing catalyst 7. Due to the above feedbackcontrol, an appropriate amount of hydrogen components can be alwayssupplied to the NO_(X) occluding and reducing catalyst 7 in the processof regenerating the NO_(X) occluding and reducing catalyst 7.

FIG. 5 is a flowchart for explaining the regenerating operation of theNO_(X) occluding and reducing catalyst 7 of the present embodimentdescribed above. This operation is conducted by ECU 30 as a routine tobe executed at regular intervals.

Operation of FIG. 5 is carried out as follows. First, in step 501, it isjudged whether or not the value of the regeneration execution flag X isset at 1. In the same manner as that of the first and the secondembodiment, in the present embodiment, the value of the flag X is set at1 by the operation not shown, which is separately executed by ECU 30,when the NOX occlusion amount of the NO_(X) occluding and reducingcatalyst 7 is increased to a predetermined value.

In this connection, the value of the regenerating operation executionflag X of this embodiment may be set in such a manner that, for example,after the value is set at X=1, the value is reset to 0 after apredetermined period of time has passed. Alternatively, the value of theflag X may be reset to 0 according to the operation shown in FIGS. 3 and4.

In the case of X=1 (the execution of regenerating operation) in step501, the rich spike operation is conducted in step 503, and an amount offuel injection of the engine 1 is increased and the engine 1 is operatedat a rich air-fuel ratio. Due to the foregoing, the exhaust gas of arich air-fuel ratio containing hydrogen components flows into the NO_(X)occluding and reducing catalyst 7, and the NO_(X) occluding and reducingcatalyst 7 is regenerated.

Next, in step 505, the hydrogen component concentration HRF in theexhaust gas flowing into the NO_(X) occluding and reducing catalyst 7 isread in from the H₂ sensor 31 disposed on the upstream side of theNO_(X) occluding and reducing catalyst 7. In steps 507 to 509, an amountof fuel injection of the engine 1 during the rich spike operation, isdecreased (step 509) or increased (step 511) so that the hydrogencomponent concentration value HRP, which is actually measured, can be apredetermined target value γ.

The target value γ of the hydrogen component concentration changes bythe NO_(X) occlusion amount at the time of starting the regeneratingoperation and by the type of the NO_(X) occluding and reducing catalyst.Therefore, it is preferable that the target value γ of the hydrogencomponent concentration is determined based on experiment in which theactual NO_(X) occluding and reducing catalyst is used.

Due to the foregoing, in the present embodiment, when the NO_(X)occluding and reducing catalyst 7 is actually regenerated, anappropriate amount of hydrogen is always supplied to the NO_(X)occluding and reducing catalyst 7. Therefore, the NO_(X) occluding andreducing catalyst 7 can be appropriately regenerated.

In this connection, the regenerating operation of the NO_(X) occludingand reducing catalyst of the present embodiment, in which the upstreamside H₂ sensor 31 (shown in FIG. 1) is used, can be conductedindependently. However, as described above, it is possible to conductthe regenerating operation in which the downstream side H₂ sensor 33(shown in FIG. 1) of the first or the second embodiment is also used.

(4) FOURTH EMBODIMENT

Next, the fourth embodiment of the present invention will be explainedbelow. In this embodiment, in the same manner as that of the thirdembodiment described above, the air-fuel ratio of the exhaust gas isfeedback controlled so that the hydrogen component concentration in theexhaust gas detected by the upstream side H₂ sensor 31 becomes thetarget value γ.

The flowchart of the regenerating operation of this embodiment is thesame as that (shown in FIG. 5) of the third embodiment.

In the third embodiment described above, the target value γ of thehydrogen component concentration HRF in the exhaust gas flowing into theNO_(X) occluding and reducing catalyst 7, is set at a constant value. Onthe other hand, in the present embodiment, the target value γ is set sothat it can be changed according to the lapse of time. Therefore, at thetime of starting the regenerating operation (the rich spike operation),the target value γ is set to be high, and then the target value γ is setso that it can be lowered with the lapse of time. Only this point ofthis embodiment is different from the third embodiment.

FIG. 6 is a graph schematically showing a change in the hydrogencomponent target value γ with the time in the present embodiment. Asshown in FIG. 6, the value of γ is high at the time of starting theregenerating operation of the NO_(X) occluding and reducing catalyst 7.As time passes, the value of γ is gradually decreased.

When the regenerating operation of the NO_(X) occluding and reducingcatalyst is started and the exhaust gas of a rich air-fuel ratio flowsinto the NO_(X) occluding and reducing catalyst, first, the NO_(X),attached onto a surface of Pt and alumina on the NO_(X) occluding andreducing catalyst and the NO_(X), existing in the neighborhood of theocclusion material surface in the form of ions, are simultaneouslydesorbed and, after that, the NO_(X) occluded in the occlusion materialis moved from the inside of the occlusion material onto the surface anddesorbed.

Therefore, at the beginning of the regenerating operation of the NO_(X)occluding and reducing catalyst, a relatively large amount of NO_(X) isdesorbed in a short period of time. After that, a rate of the desorptionof NO_(X) is gradually decreased. Therefore, an amount of hydrogencomponents necessary for reducing the desorbed NO_(X) is large at thebeginning of the regenerating operation. After that, the amount ofhydrogen components necessary for reducing the thus desorbed NO_(X) isdecreased.

In the present embodiment, as shown in FIG. 6, the hydrogen componentconcentration in the exhaust gas is set in accordance with thedesorption rate of NO_(X) of desorbing from the NO_(X) occluding andreducing catalyst during the regenerating operation. Therefore, theregeneration of the NO_(X) occluding and reducing catalyst can beappropriately conducted.

(5) FIFTH EMBODIMENT

Next, the fifth embodiment of the present invention will be explainedbelow.

In the present embodiment, a degree of the deterioration of the NO_(X)occluding and reducing catalyst is judged in accordance with the timerequired for the regenerating operation. The time required for theregenerating operation, i.e., the timing for terminating theregenerating operation is determined in accordance with the output ofthe downstream side H₂ sensor 33 as explained in the first embodiment(shown in FIG. 3).

As described before, in the first embodiment (shown in FIG. 3), a pointof time, at which the regeneration of the NO_(X) occluding and reducingcatalyst 7 is completed, is detected by the downstream side hydrogensensor 33. In this connection, a necessary period of time from the startof the regeneration to the completion of the regeneration is increasedand decreased according to an amount of NO_(X) occluded in the NO_(X)occluding and reducing catalyst 7.

In each embodiment described above, according to the value of the NO_(X)counter and the output of NO_(X) sensor disposed on the downstream sideof the NO_(X) occluding and reducing catalyst, an amount of NO_(X),which is occluded in the NO_(X) occluding and reducing catalyst, isestimated, and each time this NO_(X) occlusion amount reaches apredetermined value, the regenerating operation is executed. Therefore,it is considered that the NO_(X) occlusion amount of the NO_(X)occluding and reducing catalyst 7 at the time of starting theregenerating operation should be a constant value by nature, and theperiod of time from the start of the regenerating operation to the endshould be a substantially constant.

However, as the NO_(X) occluding and reducing catalyst 7 deteriorates,the NO_(X) occlusion capacity is lowered. Therefore, for example, in thecase where the NO_(X) occlusion amount is estimated with the NO_(X)counter, even if the NO_(X) component concentration in the exhaust gasis constant, as the NO_(X) occluding and reducing catalyst deteriorates,an amount of NO_(X), which is occluded in the NO_(X) occluding andreducing catalyst per unit time, is decreased and an actual amount ofNO_(X), which is occluded in the NO_(X) occluding and reducing catalystis decreased to be smaller than the value of the NO_(X) counter.

In the case where the NO_(X) sensor is disposed on the downstream sideof the NO_(X) occluding and reducing catalyst and the NO_(X) occlusionamount is estimated, when the occlusion capacity of the NO_(X) occludingand reducing catalyst is deteriorated, a smaller amount of NO_(X) flowsout on the downstream side of the occluded NO_(X) than an amount whichflows out of the occluded NO_(X) of the catalyst which is notdeteriorated.

Therefore, in the case where the catalyst is deteriorated, an amount ofoccluded NO_(X) of the NO_(X) occluding and reducing catalyst isdecreased at the time of starting the regenerating operation in casewhere the regenerating operation is started according to the NO_(X)counter and in the case where the regenerating operation is startedaccording to an output of the NO_(X) sensor.

The required time from the start of the regenerating operation to theend is increased and decreased according to an amount of the occludedNO_(X) of the NO_(X) occluding and reducing catalyst 7. Therefore, whenthe amount of the occluded NO_(X) of the NO_(X) occluding and reducingcatalyst 7 is decreased at the time of starting the regeneratingoperation, the required time from the start of the regeneratingoperation to the end is shortened. That is, the required time necessaryfor the regenerating operation is shortened as the catalystdeteriorates.

In the present embodiment, the above fact is utilized. When the requiredtime from the start of the regenerating operation to the end becomesshorter than a predetermined period of time, it is judged that theNO_(X) occluding and reducing catalyst 7 has been deteriorated.

FIG. 7 is a flowchart for explaining a deterioration judgment operationof the present embodiment. This operation is conducted by ECU 30 as aroutine to be executed at regular intervals.

In this embodiment, a period of time, from a point of time at which thevalue of the regenerating operation executing flag X used in the firstembodiment (shown in FIG. 3) is changed from 0 to 1 (the start of theregenerating operation) to a point of time at which the value of theregenerating operation executing flag X is changed from 1 to 0, ismeasured and, when this value becomes lower than a predetermined value,it is judged that the NO_(X) occluding and reducing catalyst 7 isdeteriorated.

That is, in the operation shown in FIG. 7, first, in step 701, it isjudged whether or not the value of the flag X is set at 1 at present. Asdescribed before, by the operation separately executed by ECU 30, theflag X is set at 1 when the regenerating operation is started.

In the case of X=1 in step 701, step 703 is executed next, and the valueof the judgment execution flag XS is set at 1 and the value of theregeneration time counter CN is increased by Δt in step 705.

In this case, the judgment execution flag XS is a flag for executing thejudgment operation of step 711 to step 715 only once after thecompletion of the regenerating operation.

The regeneration time counter CN is a counter to express the lapse oftime from the start of the regenerating operation. In this case, acounter increment Δt in step 705 is an interval (time) of repetition ofthe operation shown in FIG. 7. In the case where the regeneratingoperation is not conducted, the value of the counter CN is alwayscleared in step 717. Therefore, the value of the counter CN calculatedin step 705 is equal to the lapse of time from the establishment of X=1in step 701.

In the case where X≠1 in step 701, that is, in the case where theregeneration of the NO_(X) occluding and reducing catalyst 7 is notbeing conducted, the program proceeds to step 707, and it is judgedaccording to the value of the flag XS whether or not the execution ofthe operation of this time is the first execution from the completion ofthe regenerating operation (from the time of X≠0).

In the case of X=1, the flag XS is always set at XS=1 in step 703. Atthe time of the first execution of the operation after X≠1, the flag XSis set at XS=0 in step 709.

Therefore, in the case where XS=1 in step 707, it is the first executionof the operation from the completion of the regenerating operation.Accordingly, in this case, the value of the counter CN is equal to therequired time from the start of the regeneration of the NO_(X) occludingand reducing catalyst 7 to the end.

Therefore, in the case of X=1 in step 707, it is judged in step 711whether or not the required time CN of the regenerating operation isshorter than the predetermined judgment value δ.

In the case of CN<δ, it can be judged that the NO_(X) occlusion capacityis decreased due to the deterioration of the catalyst and the requiredtime of the regenerating operation is shortened. Therefore, the value ofthe deterioration flag XF is set at 1 (deterioration) in step 713.

In the case where CN≧δ in step 711, the value of the flag XF is set at 0(normal) in step 715.

In this case, δ is the required time for the regenerating operation inthe case where the NO_(X) occlusion capacity is deteriorated by thedeterioration of the catalyst so that problems may be caused in thepractical use. Therefore, it is preferable that the value of δ is set bymaking an experiment in which the actual catalyst is used.

After the judgment has been carried out in steps 711 to 715, the valueof the counter CN is cleared and the operation of this time isterminated. In this connection, as the value of XS is set at 0 in step709, the judgment operation of steps 711 to 715 is conducted only onceimmediately after the completion of the regeneration of the NO_(X)occluding and reducing catalyst 7. After that, step 717 is directlycarried out after step 707.

As described above, according to the present embodiment, while theregenerating operation of the NO_(X) occluding and reducing catalyst 7is being appropriately carried out, it is possible to accurately judgewhether or not the NO_(X) occluding and reducing catalyst 7 hasdeteriorated.

The deterioration judgment operation of this embodiment is carried outtogether with the regenerating operation of the first embodiment (shownin FIG. 3). Further, when the third embodiment (shown in FIG. 5)described before is simultaneously carried out and a hydrogen componentconcentration contained in the exhaust gas flowing into the NO_(X)occluding and reducing catalyst 7 in the process of the regeneratingoperation is controlled to be a predetermined value, the deteriorationjudgment accuracy can be further enhanced.

(6) SIXTH EMBODIMENT

Next, the sixth embodiment of the present invention will be explainedbelow.

FIG. 8 is the same view as FIG. 1 showing an arrangement of the presentembodiment.

The arrangement of FIG. 8 is different from that of FIG. 1 on thefollowing points. In the arrangement of FIG. 8, a so-called tandem typeNO_(X) occluding and reducing catalyst is used in which two NO_(X)occluding and reducing catalysts 71, 73 are arranged in series with eachother in the exhaust gas passage instead of the NO_(X) occluding andreducing catalyst 7 of FIG. 1.

The tandem type NO_(X) occluding and reducing catalyst of thisembodiment is composed in such a manner that the front stage NO_(X)occluding and reducing catalyst 71 and the rear stage NO_(X) occludingand reducing catalyst 73 are arranged in the casing 70 while leaving anappropriate interval between them. In the space formed between the frontstage and the rear stage, H₂ sensor 35, which is the same as H₂ sensors31, 33 shown in FIG. 2, is arranged.

In the tandem type NO_(X) occluding and reducing catalyst, when thecharacteristic of the NO_(X) occluding and reducing catalyst of thefront stage is made to be different from that of the NO_(X) occludingand reducing catalyst of the rear stage, the exhaust gas purifyingperformance of the tandem type NO_(X) occluding and reducing catalyst isenhanced from that of a single stage NO_(X) occluding and reducingcatalyst.

For example, in the tandem type NO_(X) occluding and reducing catalystof the present embodiment, the NO_(X) occlusion capacity of the frontstage NO_(X) occluding and reducing catalyst 71 is larger than that ofthe rear stage NO_(X) occluding and reducing catalyst 73, and the O₂storage capacity of the front stage NO_(X) occluding and reducingcatalyst 71 is smaller than that of the rear stage NO_(X) occluding andreducing catalyst 73. Further, the catalyst supporting amount of Pt ofthe front stage NO_(X) occluding and reducing catalyst 71 is larger thanthat of the rear stage NO_(X) occluding and reducing catalyst 73.

In the case where two NO_(X) occluding and reducing catalysts arearranged in series to each other, NO_(X) is first occluded in the frontstage NO_(X) occluding and reducing catalyst. As an O₂ storage capacityof the front stage is set at a small value, even at the time of carryingout the regenerating operation, the hydrogen components and HC and COcomponents contained in the exhaust gas do not react with oxygenoccluded in the catalyst and most of the hydrogen components and HC andCO components are used for reducing NO_(X).

As the amount of Pt carried on the catalyst in the front stage isincreased, most of NO contained in the exhaust gas is oxidized on thefront stage catalyst and changed into NO₂. Therefore, an amount ofocclusion of NO_(X) per unit volume at the front stage is increased.Therefore, in addition to the setting in which the occlusion capacity ofthe front stage NO_(X) occluding and reducing catalyst is increased, theocclusion and the reducing purification of NO_(X) can be effectivelyaccomplished in the front stage NO_(X) occluding and reducing catalyst71.

On the other hand, in the rear stage NO_(X) occluding and reducingcatalyst 73, the O₂ storage capacity is set relatively high. Therefore,for example, even in the case where the air-fuel ratio of the exhaustgas passing through the front stage NO_(X) occluding and reducingcatalyst at the time of carrying out the regenerating operation becomesrich, the rear stage NO_(X) occluding and reducing catalyst 73 can bemaintained in an atmosphere close to the stoichiometric air-fuel ratio.As the NO_(X) occluding and reducing catalyst has a function of thethree way catalyst in the neighborhood of the stoichiometric air-fuelratio, in the tandem type NO_(X) occluding and reducing catalyst, at thetime of the regeneration, the rear stage NO_(X) occluding and reducingcatalyst 73 functions as a three way catalyst using oxygen dischargedfrom the O₂ storage. Therefore, even when NO_(X), which has beendesorbed at the front stage, is not reduced for some reason and flowsinto the rear stage NO_(X) occluding and reducing catalyst 73, NO_(X)can be reduced and purified on the rear stage NO_(X) occluding andreducing catalyst 73.

As described above, in the tandem type NO_(X) occluding and reducingcatalyst, the NO_(X) occluding and reducing catalysts, thecharacteristics of which are different from each other, are separatelyarranged at the front stage and the rear stage, so that the exhaust gaspurifying efficiency can be enhanced. However, as a result of adoptingthe above structure, in the case where the same control as that of eachembodiment described before is conducted, if H₂ sensor is disposed onthe downstream side of the rear stage NO_(X) occluding and reducingcatalyst 73, problems may be caused.

For example, in the case where the time for terminating the regeneratingoperation is judged according to an output of H₂ sensor in the samemanner as that of the first embodiment, if H₂ sensor is disposed on thedownstream side of the rear stage NO_(X) occluding and reducing catalyst73, it is difficult to judge the time for terminating the regeneratingoperation.

The reason is described as follows. In the tandem type NO_(X) occludingand reducing catalyst described before, the rear stage NO_(X) occludingand reducing catalyst 73 has a relatively large O₂ storage capacity.Therefore, even when the front stage NO_(X) occluding and reducingcatalyst 71 completes the regenerating operation at the time ofexecuting the regenerating operation and the hydrogen components arecontained in the exhaust gas at the outlet of the front stage NO_(X)occluding and reducing catalyst 71, these hydrogen components react withoxygen, which is discharged from the rear stage NO_(X) occluding andreducing catalyst 73, when these hydrogen components pass through therear stage NO_(X) occluding and reducing catalyst 73. Accordingly, thesehydrogen components do not flow out onto the downstream side of the rearstage NO_(X) occluding and reducing catalyst 73.

Accordingly, when an H₂ sensor is disposed on the downstream side of therear stage NO_(X) occluding and reducing catalyst 73 and the time forterminating the regenerating operation is judged based on the output ofthe H₂ sensor, it is difficult to judge the completion of theregenerating operation accurately unless all oxygen occluded in the rearstage NO_(X) occluding and reducing catalyst 73 is discharged.

However, actually, in the tandem NO_(X) occluding and reducing catalyst,almost all of the occlusion and the reducing purification of NO_(X) areconducted in the front stage NO_(X) occluding and reducing catalyst 71,and the rear stage NO_(X) occluding and reducing catalyst 73 onlyperforms an auxiliary role. Therefore, an amount of occluded NO_(X) ismuch smaller than that of the front stage. Accordingly, after thecompletion of the regeneration of the front stage NO_(X) occluding andreducing catalyst 71, it is unnecessary to continue the regeneratingoperation. When the regenerating operation is continued until thehydrogen components are detected in the exhaust gas on the downstreamside of the rear stage NO_(X) occluding and reducing catalyst, the fuelconsumption of the engine is increased.

Therefore, in the present embodiment, according to an output of H2sensor 35 which is arranged in a space formed between the front stageNO_(X) occluding and reducing catalyst 71 and the rear stage NO_(X)occluding and reducing catalyst 73, the time of the completion of theregenerating operation of the front stage NO_(X) occluding and reducingcatalyst 71 is judged so as to finish the regenerating operation.

As the regenerating operation of this embodiment is basically the sameas that of the first embodiment (shown in FIG. 3), detailed explanationsare omitted here.

In this connection, when the regenerating operation of the tandem typeNO_(X) occluding and reducing catalyst is terminated when theregenerating operation of the front stage NO_(X) occluding and reducingcatalyst 71 is completed as described in this embodiment, it appearsthat an amount of the NO_(X) occluded in the rear stage NO_(X) occludingand reducing catalyst 73 is increased while it is not being regenerated.However, as described before, the amount of the NO_(X) occluded in therear stage NO_(X) occluding and reducing catalyst 73 is so small that itis unnecessary to frequently conduct the regenerating operation of therear stage NO_(X) occluding and reducing catalyst 73. Practically, noproblems are caused when the regenerating operation is conducted in sucha manner that the engine 1 is continuously operated at thestoichiometric air-fuel ratio for a certain period of time and duringwhich period the regenerating operation of the rear stage NO_(X)occluding and reducing catalyst 73 is naturally conducted.

(7) SEVENTH EMBODIMENT

As described before, in the NO_(X) occluding and reducing catalyst, whenSO_(X) is contained in the exhaust gas flowing into the NO_(X) occludingand reducing catalyst, SO_(X) is occluded in the NO_(X) occluding andreducing catalyst simultaneously with NO_(X) under the condition of alean air-fuel ratio.

In this case, NO_(X) occluded in the NO_(X) occluding and reducingcatalyst can be relatively simply desorbed from the NO_(X) occluding andreducing catalyst 7 by carrying out the regenerating operation, however,as the affinity of SO_(X) with the occluded NO_(X) is strong and astable chemical compound is generated, when SO_(X) is once occluded inthe NO_(X) occluding and reducing catalyst, SO_(X) cannot be desorbedfrom the NO_(X) occluding and reducing catalyst by only a simpleregenerating operation of the NO_(X) occluding and reducing catalyst.Therefore, SO_(X) is gradually accumulated in the catalyst and theNO_(X) occluding and reducing catalyst is affected by SO_(X), that is,SO_(X) poisoning is caused.

Therefore, usually, when the NO_(X) occluding and reducing catalyst isused, each time an amount of SO_(X) occluded in the catalyst isincreased to a certain value, the poisoning regeneration treatment isconducted so as to desorb SO_(X) from the NO_(X) occluding and reducingcatalyst.

In the poisoning regeneration treatment, the engine is operated underthe operating condition of a high exhaust gas temperature at a richair-fuel ratio so as to maintain the NO_(X) occluding and reducingcatalyst at a high temperature in a rich air-fuel ratio atmosphere. Evenin this case, when hydrogen components are contained in the exhaust gas,the poisoning regeneration treatment time can be greatly shortened.

In the poisoning regeneration treatment, the generated sulfate isdecomposed when the NO_(X) occluding and reducing catalyst temperatureis raised, so as to desorb SO_(X) from the catalyst. When the NO_(X)occluding and reducing catalyst is maintained at a rich air-fuel ratio,SO_(X), which has been desorbed, is prevented from being occluded againin the NO_(X) occluding and reducing catalyst. However, actually, as theaffinity of SO_(X) with the occluded NO_(X) is strong as describedbefore, even if the air-fuel ratio is maintained to be rich, SO_(X),which has been desorbed from the upstream portion of the NO_(X)occluding and reducing catalyst, is occluded again in the downstreamportion. Therefore, while repeating desorption and occlusion, the SO_(X)desorbed from the upstream portion of the NO_(X) occluding and reducingcatalyst gradually moves onto the downstream side. Accordingly, it takesa relatively long period of time for SO_(X) to be completely desorbedfrom the catalyst.

Therefore, the poisoning regeneration treatment requires a relativelylong period of time and deteriorates the fuel consumption of the engine.Further, the NO_(X) occluding and reducing catalyst is exposed to a hightemperature over a long period of time, which deteriorates the catalyst.

However, as the reducing power of hydrogen is very strong, itfacilitates the desorption of SO_(X) from NO_(X) occluding and reducingcatalyst, and further hydrogen reacts with SO_(X), which has been oncedesorbed, and prevents SO_(X), which has been once desorbed, from beingoccluded again into the NO_(X) occluding and reducing catalyst.Therefore, when H₂ is supplied to the NO_(X) occluding and reducingcatalyst at the time of poisoning regeneration treatment, SO_(X) can becompletely desorbed from the NO_(X) occluding and reducing catalyst in ashort period of time.

In this case, in the same manner as that of the regenerating operationof the NO_(X) occluding and reducing catalyst, while SO_(X) is beingdesorbed from the NO_(X) occluding and reducing catalyst, the hydrogencomponents contained in the exhaust gas are consumed by reacting withSO_(X). Therefore, the hydrogen components do not flow out onto thedownstream side of the NO_(X) occluding and reducing catalyst. After allthe SO_(X) has been desorbed, that is, after the poisoning regenerationtreatment has been completed, the hydrogen components flow out onto thedownstream side of the NO_(X) occluding and reducing catalyst for thefirst time.

Therefore, by the same method as that of judging the time forterminating the regenerating operation, the time for terminating thepoisoning regeneration treatment can be accurately judged.

However, in the case where the tandem type NO_(X) occluding and reducingcatalyst is used, when an H₂ sensor is disposed on the downstream sideof the rear stage NO_(X) occluding and reducing catalyst, the sameproblems as those caused in the judgment of the time for terminating theregenerating operation may occur. Therefore, it is difficult toaccurately judge the time for terminating the poisoning regenerationtreatment. Accordingly, such a problem may be caused that, although thepoisoning regeneration treatment is not actually completed, the exhaustgas of a high temperature is supplied to the NO_(X) occluding andreducing catalyst at a rich air-fuel ratio over a long period of time.Therefore, when H₂ sensor is disposed on the downstream side of the rearstage NO_(X) occluding and reducing catalyst so as to judge the time forterminating the poisoning regeneration treatment of the tandem typeNO_(X) occluding and reducing catalyst, the fuel consumption of theengine is increased due to the unnecessary rich air-fuel operation ofthe engine. Further, when the catalyst is exposed to a high temperatureover a long period of time, the NO_(X) occluding and reducing catalystis deteriorated.

In order to increase the O₂ storage capacity, the rear stage NO_(X)occluding and reducing catalyst carries a relatively large amount ofceria (Ce). Although ceria is easily bonded with SO_(X) and sulfate iseasily formed, the bonding strength of ceria with SO_(X) is so weak thatSO_(X) can be easily desorbed in a short period of time during thepoisoning regeneration treatment.

Therefore, in this embodiment, the time for terminating the poisoningregeneration is judged by using H₂ sensor 35 (shown in FIG. 8) arrangedin a space between the front stage and the rear stage.

FIG. 9 is a flowchart for explaining the poisoning regenerationtreatment of this embodiment. The operation shown in FIG. 9 is basicallythe same as that of the flow chat of the regenerating operation of thefirst embodiment (shown in FIG. 3). The operation of FIG. 9 is differentfrom operation of FIG. 3 only at the following three points. First,instead of the flag X, the poisoning regeneration treatment executionflag S is used. Secondly, in step 903, instead of the rich spikeoperation of step 303 of FIG. 3, the poisoning regenerating operation isexecuted in which the engine conducts a rich air-fuel ratio operationunder the condition of a high exhaust gas temperature. Thirdly, in steps905, 907, an output HRM of the H₂ sensor 35, which is arranged betweenthe front stage and the rear stage, is used. Therefore, detailedexplanations of FIG. 9 are omitted here.

As described above, the time for terminating the poisoning regenerationtreatment is judged according to the hydrogen component concentrationdetected by the H₂ sensor arranged between the front stage NO_(X)occluding and reducing catalyst and the rear stage NO_(X) occluding andreducing catalyst. Due to the foregoing, it is possible to prevent anincrease in the fuel consumption and a deterioration of the NO_(X)occluding and reducing catalyst.

(8) EIGHTH EMBODIMENT

Next, the eighth embodiment of the present invention will be explainedbelow.

In the present embodiment, when the time for terminating theregenerating operation of the NO_(X) occluding and reducing catalyst isjudged according to an output of the H₂ sensor 35 arranged between thefront stage and the rear stage of the tandem type NO_(X) occluding andreducing catalyst shown in FIG. 8, a degree of deterioration of thefront stage NO_(X) occluding and reducing catalyst 71 is judgedaccording to the required time from the start of the regeneratingoperation to the end.

In this connection, the specific method of judging a degree ofdeterioration is substantially the same as that of the fifth embodimentshown in FIG. 7. Therefore, detailed explanations are omitted here.

As described before, in the tandem type NO_(X) occluding and reducingcatalyst, the occlusion and the reducing purification of NO_(X) aremainly conducted by the front stage NO_(X) occluding and reducingcatalyst 71. Therefore, it is necessary to accurately judge the degreeof deterioration of the front stage NO_(X) occluding and reducingcatalyst 71.

In the present embodiment, the degree of deterioration of the frontstage NO_(X) occluding and reducing catalyst 71 is judged according toan output of the H₂ sensor 35 arranged between the front stage and therear stage of the tandem type NO_(X) occluding and reducing catalyst.Due to the foregoing, while the tandem type NO_(X) occluding andreducing catalyst is being appropriately regenerated, the degree ofdeterioration of the front stage NO_(X) occluding and reducing catalyst71 can be judged.

1. An exhaust gas purifying device for an internal combustion enginecomprising: an NO_(X) occluding and reducing catalyst disposed in anexhaust passage of an internal combustion engine, the NO_(X) occludingand reducing catalyst occluding NO_(X) contained in exhaust gas by oneof the absorption and the adsorption or by both the absorption and theadsorption when an air-fuel ratio of the exhaust gas flowing into thecatalyst is lean, and reducing and purifying the occluded NO_(X) with areducing component contained in the exhaust gas when an air-fuel ratioof the exhaust gas is a stoichiometric air-fuel ratio or a rich air-fuelratio; and an H₂ sensor disposed in at least one of the exhaust gaspassages on the inlet side and the outlet side of the NO_(X) occludingand reducing catalyst for detecting a hydrogen component concentrationof the exhaust gas, wherein the exhaust gas purifying device executes aregenerating operation in which the exhaust gas of a rich air-fuel ratioor a stoichiometric air-fuel ratio is supplied to the NO_(X) occludingand reducing catalyst for a predetermined period of time when the NO_(X)occluding and reducing catalyst is to reduce and purify the NO_(X)occluded in the NO_(X) occluding and reducing catalyst and, during theregenerating operation, the exhaust gas purifying device controls theair-fuel ratio of the exhaust gas flowing into the NO_(X) occluding andreducing catalyst based on the hydrogen component concentration, in theexhaust gas, detected by the H₂ sensor.
 2. An exhaust gas purifyingdevice for an internal combustion engine according to claim 1, whereinthe H₂ sensor is arranged in an exhaust gas passage on an outlet side ofthe NO_(X) occluding and reducing catalyst, and at the time of executingthe regenerating operation, a time for terminating the regeneratingoperation is judged according to a hydrogen component concentration inthe exhaust gas detected by the outlet side H₂ sensor.
 3. An exhaust gaspurifying device for an internal combustion engine according to claim 1,wherein the H₂ sensor is arranged in at least an exhaust gas passage onthe outlet side of the NO_(X) occluding and reducing catalyst, theregenerating operation includes operations for first supplying theexhaust gas of a rich air-fuel ratio to the NO_(X) occluding andreducing catalyst and then supplying the exhaust gas of a stoichiometricair-fuel ratio to the NO_(X) occluding and reducing catalyst and, thetime at which the exhaust gas air-fuel ratio is switched from the richair-fuel ratio to the stoichiometric air-fuel ratio is determined inaccordance with the hydrogen component concentration detected by theoutlet side H₂ sensor.
 4. An exhaust gas purifying device for aninternal combustion engine according to claim 1, wherein the H₂ sensoris arranged in an exhaust gas passage on the inlet side of the NO_(X)occluding and reducing catalyst, and at the time of executing theregenerating operation, an air-fuel ratio of the exhaust gas flowinginto the NO_(X) occluding and reducing catalyst is controlled so that ahydrogen component concentration in the exhaust gas detected by theinlet side H₂ sensor can be a predetermined target value.
 5. An exhaustgas purifying device for an internal combustion engine according toclaim 4, wherein the target value of the hydrogen componentconcentration is high at the time of starting the regeneratingoperation, and then the target value of the hydrogen componentconcentration is gradually decreased with the lapse of time.
 6. Anexhaust gas purifying device for an internal combustion engine accordingto claim 2, wherein according to the lapse of time from the start of theregenerating operation to the end of the regenerating operation which isjudged according to the hydrogen component concentration in the exhaustgas detected by the downstream side H₂ sensor, a degree of deteriorationof the NO_(X) occluding and reducing catalyst is judged.
 7. An exhaustgas purifying device for an internal combustion engine comprising anNO_(X) occluding and reducing catalyst disposed in an exhaust passage ofan internal combustion engine, the NO_(X) occluding and reducingcatalyst occluding NO_(X) contained in exhaust gas by one of theabsorption and the adsorption or by both the absorption and theadsorption when an air-fuel ratio of the exhaust gas flowing into thecatalyst is lean and reducing and purifying the occluded NO_(X) with areducing component contained in the exhaust gas when an air-fuel ratioof the exhaust gas is a stoichiometric air-fuel ratio or a rich air-fuelratio, the exhaust gas purifying device for an internal combustionengine further comprising: NO_(X) occluding and reducing catalystsarranged in series to each other on the upstream side and the downstreamside of the exhaust gas passage of the internal combustion engine; andan H₂ sensor, which is arranged in series to each other in the exhaustgas passage between the upstream side NO_(X) occluding and reducingcatalyst and the downstream side NO_(X) occluding and reducing catalyst,for detecting a hydrogen component concentration in the exhaust gas,wherein when the NO_(X) occluding and reducing catalyst is to reduce andpurify NO_(X) occluded during a lean air-fuel ratio operation of theengine, at the time of executing a regenerating operation in which theexhaust gas of a rich air-fuel ratio or a stoichiometric air-fuel ratiois supplied to the NO_(X) occluding and reducing catalyst for apredetermined period of time, according to the hydrogen componentconcentration in the exhaust gas detected by the H₂ sensor, an air-fuelratio of the exhaust gas flowing into the upstream side NO_(X) occludingand reducing catalyst is controlled.
 8. An exhaust gas purifying devicefor an internal combustion engine according to claim 7, wherein anNO_(X) occlusion capacity of the upstream side NO_(X) occluding andreducing catalyst is larger than that of the downstream side NO_(X)occluding and reducing catalyst, an O₂ storage capacity of the upstreamside NO_(X) occluding and reducing catalyst is smaller than that of thedownstream side NO_(X) occluding and reducing catalyst, and an amount ofplatinum components carried on the upstream side NO_(X) occluding andreducing catalyst is larger than that carried on the downstream sideNO_(X) occluding and reducing catalyst.
 9. An exhaust gas purifyingdevice for an internal combustion engine according to claim 7 or 8,wherein a time for terminating the regenerating operation is judgedaccording to the hydrogen component concentration in the exhaust gasdetected by the H₂ sensor at the time of executing the regeneratingoperation.
 10. An exhaust gas purifying device for an internalcombustion engine according to claim 7 or 8, wherein the device furtherexecutes a poisoning regeneration treatment in order to desorb sulfuroxide occluded in the NO_(X) occluding and reducing catalyst togetherwith NO_(X) from the NO_(X) occluding and reducing catalyst by makingthe air-fuel ratio of the exhaust gas flowing into the NO_(X) occludingand reducing catalyst to be a rich air-fuel ratio and, at the same time,raising the temperature thereof and, wherein an air-fuel ratio of theexhaust gas flowing into the upstream side NO_(X) occluding and reducingcatalyst is controlled according to the hydrogen component concentrationin the exhaust gas detected by the H₂ sensor in the process of executingthe poisoning regeneration treatment.
 11. An exhaust gas purifyingdevice for an internal combustion engine, according to claim 10, whereina time for terminating the poisoning regeneration treatment is judgedaccording to the hydrogen component concentration in the exhaust gasdetected by the H₂ sensor at the time of executing the poisoningregeneration treatment.
 12. An exhaust gas purifying device for aninternal combustion engine according to claim 8, wherein a degree ofdeterioration of the upstream side NO_(X) occluding and reducingcatalyst is judged according to a period of time from the start of theregenerating operation to the end of the regenerating operation which isjudged according to the hydrogen component concentration, in the exhaustgas, detected by the H₂ sensor.